During python startup, pthread mutex locks and unlocks are called several times from a stack like this:
(gdb) bt
#0  0xb7f3e536 in pthread_mutex_lock () from /lib/tls/i686/cmov/libpthread.so.0
#1  0xb7f685ed in _dl_lookup_symbol_x () from /lib/ld-linux.so.2
#2  0xb7f69710 in _dl_relocate_object () from /lib/ld-linux.so.2
#3  0xb7f700bb in dl_open_worker () from /lib/ld-linux.so.2
#4  0xb7f6c016 in _dl_catch_error () from /lib/ld-linux.so.2
#5  0xb7f6f99e in _dl_open () from /lib/ld-linux.so.2
#6  0xb7f33c19 in ?? () from /lib/tls/i686/cmov/libdl.so.2
#7  0xbf999f97 in ?? ()
#8  0x80000002 in ?? ()
#9  0x080f101c in _PyImport_GetDynLoadFunc (
    fqname=0xbf999c30 "\227\237\231¿\002", shortname=0x0, 
    pathname=0xb7c3b630 "U\211åS\203ì\004è", fp=0xb7c33b22)
    at ../Python/dynload_shlib.c:134
#10 0xb7f6c016 in _dl_catch_error () from /lib/ld-linux.so.2
#11 0xb7f342bc in ?? () from /lib/tls/i686/cmov/libdl.so.2
#12 0xb7f33b80 in ?? () from /lib/tls/i686/cmov/libdl.so.2
#13 0xbf999c30 in ?? ()
#14 0xb7f33b80 in ?? () from /lib/tls/i686/cmov/libdl.so.2
#15 0xb7f35ff4 in ?? () from /lib/tls/i686/cmov/libdl.so.2

then while loading pdb:

Breakpoint 4, 0xb7f3e536 in pthread_mutex_lock ()
   from /lib/tls/i686/cmov/libpthread.so.0
(gdb) bt
#0  0xb7f3e536 in pthread_mutex_lock () from /lib/tls/i686/cmov/libpthread.so.0
#1  0xb7f33d65 in dlsym () from /lib/tls/i686/cmov/libdl.so.2
#2  0x080f104d in _PyImport_GetDynLoadFunc (fqname=0xbf991097 "strop", 
    shortname=0xbf991097 "strop", 
    pathname=0xbf990027 "/usr/lib/python2.5/lib-dynload/strop.so", 
    fp=0x81d6e08) at ../Python/dynload_shlib.c:145
#3  0x080e1a0a in _PyImport_LoadDynamicModule (name=0xbf991097 "strop", 
    pathname=0xbf990027 "/usr/lib/python2.5/lib-dynload/strop.so", 
    fp=0x81d6e08) at ../Python/importdl.c:42
#4  0x080df6d5 in load_module (name=0xbf991097 "strop", fp=0xbf98fe72, 
    buf=0xbf990027 "/usr/lib/python2.5/lib-dynload/strop.so", type=3, 
    loader=0x81e91a0) at ../Python/import.c:1758
#5  0x080df98b in import_submodule (mod=0x8146678, subname=0xbf991097 "strop", 
    fullname=0xbf991097 "strop") at ../Python/import.c:2400
#6  0x080dfe3e in load_next (mod=0x8146678, altmod=0x8146678, 
    p_name=<value optimized out>, buf=0xbf991097 "strop", p_buflen=0xbf991090)
    at ../Python/import.c:2220
#7  0x080e0068 in import_module_level (name=0x0, globals=0xb7d662b4, 
    locals=<value optimized out>, fromlist=0xb7d64e64, level=-1)
    at ../Python/import.c:2001
#8  0x080e04f7 in PyImport_ImportModuleLevel (name=0xb7d66274 "strop", 
    globals=0xb7d4edfc, locals=0xb7d4edfc, fromlist=0xb7d64e64, level=-1)
    at ../Python/import.c:2072
#9  0x080c1db4 in builtin___import__ (self=0x0, args=0xb7d67144, kwds=0x0)
    at ../Python/bltinmodule.c:47
#10 0x0805c9e7 in PyObject_Call (func=0x81e91a0, arg=0xb7d67144, kw=0x0)
    at ../Objects/abstract.c:1860
#11 0x080c232c in PyEval_CallObjectWithKeywords (func=0xb7d86e6c, 
    arg=0xb7d67144, kw=0x0) at ../Python/ceval.c:3433
#12 0x080c4a9d in PyEval_EvalFrameEx (f=0x81d4d74, throwflag=0)
    at ../Python/ceval.c:2063
#13 0x080ca115 in PyEval_EvalCodeEx (co=0xb7d654e8, globals=0xb7d4edfc, 
    locals=0xb7d4edfc, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, 
    defcount=0, closure=0x0) at ../Python/ceval.c:2831
#14 0x080ca187 in PyEval_EvalCode (co=0xb7d654e8, globals=0xb7d4edfc, 
    locals=0xb7d4edfc) at ../Python/ceval.c:494
#15 0x080de9ac in PyImport_ExecCodeModuleEx (name=0xbf9944c7 "string", 
    co=0xb7d654e8, pathname=0xbf992417 "/usr/lib/python2.5/string.pyc")
    at ../Python/import.c:675
#16 0x080df306 in load_source_module (name=0xbf9944c7 "string", 
    pathname=0xbf992417 "/usr/lib/python2.5/string.pyc", 
    fp=<value optimized out>) at ../Python/import.c:959
#17 0x080df98b in import_submodule (mod=0x8146678, 
    subname=0xbf9944c7 "string", fullname=0xbf9944c7 "string")
    at ../Python/import.c:2400
#18 0x080dfe3e in load_next (mod=0x8146678, altmod=0x8146678, 
    p_name=<value optimized out>, buf=0xbf9944c7 "string", p_buflen=0xbf9944c0)
    at ../Python/import.c:2220
#19 0x080e0068 in import_module_level (name=0x0, globals=0xb7d60ed4, 
    locals=<value optimized out>, fromlist=0x8146678, level=-1)
    at ../Python/import.c:2001
#20 0x080e04f7 in PyImport_ImportModuleLevel (name=0xb7d60e54 "string", 
    globals=0xb7d4ed74, locals=0xb7d4ed74, fromlist=0x8146678, level=-1)
    at ../Python/import.c:2072
#21 0x080c1db4 in builtin___import__ (self=0x0, args=0xb7d5be8c, kwds=0x0)
    at ../Python/bltinmodule.c:47
#22 0x0805c9e7 in PyObject_Call (func=0x81e91a0, arg=0xb7d5be8c, kw=0x0)
    at ../Objects/abstract.c:1860
#23 0x080c232c in PyEval_CallObjectWithKeywords (func=0xb7d86e6c, 
    arg=0xb7d5be8c, kw=0x0) at ../Python/ceval.c:3433
#24 0x080c4a9d in PyEval_EvalFrameEx (f=0x81d31bc, throwflag=0)
    at ../Python/ceval.c:2063
#25 0x080ca115 in PyEval_EvalCodeEx (co=0xb7d617b8, globals=0xb7d4ed74, 
    locals=0xb7d4ed74, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, 
    defcount=0, closure=0x0) at ../Python/ceval.c:2831
#26 0x080ca187 in PyEval_EvalCode (co=0xb7d617b8, globals=0xb7d4ed74, 
    locals=0xb7d4ed74) at ../Python/ceval.c:494
---Type <return> to continue, or q <return> to quit---
#27 0x080de9ac in PyImport_ExecCodeModuleEx (name=0xbf9978f7 "cmd", 
    co=0xb7d617b8, pathname=0xbf995847 "/usr/lib/python2.5/cmd.pyc")
    at ../Python/import.c:675
#28 0x080df306 in load_source_module (name=0xbf9978f7 "cmd", 
    pathname=0xbf995847 "/usr/lib/python2.5/cmd.pyc", fp=<value optimized out>)
    at ../Python/import.c:959
#29 0x080df98b in import_submodule (mod=0x8146678, subname=0xbf9978f7 "cmd", 
    fullname=0xbf9978f7 "cmd") at ../Python/import.c:2400
#30 0x080dfe3e in load_next (mod=0x8146678, altmod=0x8146678, 
    p_name=<value optimized out>, buf=0xbf9978f7 "cmd", p_buflen=0xbf9978f0)
    at ../Python/import.c:2220
#31 0x080e0068 in import_module_level (name=0x0, globals=0xb7d601b4, 
    locals=<value optimized out>, fromlist=0x8146678, level=-1)
    at ../Python/import.c:2001
#32 0x080e04f7 in PyImport_ImportModuleLevel (name=0xb7dacaf4 "cmd", 
    globals=0xb7d4e824, locals=0xb7d4e824, fromlist=0x8146678, level=-1)
    at ../Python/import.c:2072
#33 0x080c1db4 in builtin___import__ (self=0x0, args=0xb7d5b964, kwds=0x0)
    at ../Python/bltinmodule.c:47
#34 0x0805c9e7 in PyObject_Call (func=0x81e91a0, arg=0xb7d5b964, kw=0x0)
    at ../Objects/abstract.c:1860
#35 0x080c232c in PyEval_CallObjectWithKeywords (func=0xb7d86e6c, 
    arg=0xb7d5b964, kw=0x0) at ../Python/ceval.c:3433
#36 0x080c4a9d in PyEval_EvalFrameEx (f=0x81d0fd4, throwflag=0)
    at ../Python/ceval.c:2063
#37 0x080ca115 in PyEval_EvalCodeEx (co=0xb7d61140, globals=0xb7d4e824, 
    locals=0xb7d4e824, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, 
    defcount=0, closure=0x0) at ../Python/ceval.c:2831
#38 0x080ca187 in PyEval_EvalCode (co=0xb7d61140, globals=0xb7d4e824, 
    locals=0xb7d4e824) at ../Python/ceval.c:494
#39 0x080de9ac in PyImport_ExecCodeModuleEx (name=0xbf99ad27 "pdb", 
    co=0xb7d61140, pathname=0xbf998c77 "/usr/lib/python2.5/pdb.pyc")
    at ../Python/import.c:675
#40 0x080df306 in load_source_module (name=0xbf99ad27 "pdb", 
    pathname=0xbf998c77 "/usr/lib/python2.5/pdb.pyc", fp=<value optimized out>)
    at ../Python/import.c:959
#41 0x080df98b in import_submodule (mod=0x8146678, subname=0xbf99ad27 "pdb", 
    fullname=0xbf99ad27 "pdb") at ../Python/import.c:2400
#42 0x080dfe3e in load_next (mod=0x8146678, altmod=0x8146678, 
    p_name=<value optimized out>, buf=0xbf99ad27 "pdb", p_buflen=0xbf99ad20)
    at ../Python/import.c:2220
#43 0x080e0068 in import_module_level (name=0x0, globals=0xb7d9d3f4, 
    locals=<value optimized out>, fromlist=0x8146678, level=-1)
    at ../Python/import.c:2001
#44 0x080e04f7 in PyImport_ImportModuleLevel (name=0xb7d53d34 "pdb", 
    globals=0xb7d97acc, locals=0xb7d97acc, fromlist=0x8146678, level=-1)
    at ../Python/import.c:2072
#45 0x080c1db4 in builtin___import__ (self=0x0, args=0xb7d93c84, kwds=0x0)
    at ../Python/bltinmodule.c:47
#46 0x0805c9e7 in PyObject_Call (func=0x81e91a0, arg=0xb7d93c84, kw=0x0)
    at ../Objects/abstract.c:1860
#47 0x080c232c in PyEval_CallObjectWithKeywords (func=0xb7d86e6c, 
    arg=0xb7d93c84, kw=0x0) at ../Python/ceval.c:3433
#48 0x080c4a9d in PyEval_EvalFrameEx (f=0x81ce034, throwflag=0)
    at ../Python/ceval.c:2063
#49 0x080ca115 in PyEval_EvalCodeEx (co=0xb7d404e8, globals=0xb7d97acc, 
    locals=0xb7d97acc, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0, 
    defcount=0, closure=0x0) at ../Python/ceval.c:2831
#50 0x080ca187 in PyEval_EvalCode (co=0xb7d404e8, globals=0xb7d97acc, 
    locals=0xb7d97acc) at ../Python/ceval.c:494
#51 0x080e9d83 in PyRun_InteractiveOneFlags (fp=0xb7f05440, 
    filename=0x8122f24 "<stdin>", flags=0xbf99c1d8)
    at ../Python/pythonrun.c:1273
#52 0x080e9fb6 in PyRun_InteractiveLoopFlags (fp=0xb7f05440, 
    filename=0x8122f24 "<stdin>", flags=0xbf99c1d8)
    at ../Python/pythonrun.c:723
#53 0x080ea0d2 in PyRun_AnyFileExFlags (fp=0xb7f05440, 
---Type <return> to continue, or q <return> to quit---
    filename=0x8122f24 "<stdin>", closeit=0, flags=0xbf99c1d8)
    at ../Python/pythonrun.c:692
#54 0x08059390 in Py_Main (argc=0, argv=0xbf99c2a4) at ../Modules/main.c:523
#55 0x080588c2 in main (argc=-1208655872, argv=0x81d40f8)
    at ../Modules/python.c:23

However, repeated imports of a module do show futex calls in strace, but do not hit pthead_mutex_{lock,unlock} so there must be something else...