I can replicate this issue on Ubuntu 8.04 64bit. I also think it is something to  do with 64bit and Python. I have no issues on a 32bit system.

aDesklets was installed from source, using Python 2.4.

Output when launching Calendar desklet:

./Calendar.py
Do you want to (r)egister this desklet or to (t)est it? t
Now testing...
============================================================
If you do not see anything (or just an initial flicker
in the top left corner of your screen), try `--help',
and see the FAQ: `info adesklets'.
============================================================
*** glibc detected *** adesklets: double free or corruption (out): 0x0000000000651fd0 ***
======= Backtrace: =========
/lib/libc.so.6[0x7f105934c08a]
/lib/libc.so.6(cfree+0x8c)[0x7f105934fc1c]
/usr/lib/libfontconfig.so.1[0x7f1059650c2e]
/usr/lib/libfontconfig.so.1(FcPatternDestroy+0x7a)[0x7f1059650dea]
/usr/lib/libfontconfig.so.1(FcFontSetDestroy+0x27)[0x7f105964abf7]
adesklets[0x40f904]
adesklets[0x407b14]
adesklets[0x406541]
/lib/libc.so.6(__libc_start_main+0xf4)[0x7f10592f61c4]
adesklets(rl_filename_completion_function+0x5f9)[0x406479]
======= Memory map: ========
00400000-0041e000 r-xp 00000000 08:01 1191907                            /usr/local/bin/adesklets
0061d000-0061f000 rw-p 0001d000 08:01 1191907                            /usr/local/bin/adesklets
0061f000-0067e000 rw-p 0061f000 00:00 0                                  [heap]
7f1050000000-7f1050021000 rw-p 7f1050000000 00:00 0 
7f1050021000-7f1054000000 ---p 7f1050021000 00:00 0 
7f1057102000-7f105710f000 r-xp 00000000 08:01 432543                     /lib/libgcc_s.so.1
7f105710f000-7f105730f000 ---p 0000d000 08:01 432543                     /lib/libgcc_s.so.1
7f105730f000-7f1057310000 rw-p 0000d000 08:01 432543                     /lib/libgcc_s.so.1
7f1057310000-7f105731a000 r-xp 00000000 08:01 432568                     /lib/libnss_files-2.7.so
7f105731a000-7f105751a000 ---p 0000a000 08:01 432568                     /lib/libnss_files-2.7.so
7f105751a000-7f105751c000 rw-p 0000a000 08:01 432568                     /lib/libnss_files-2.7.so
7f105751c000-7f1057526000 r-xp 00000000 08:01 432578                     /lib/libnss_nis-2.7.so
7f1057526000-7f1057725000 ---p 0000a000 08:01 432578                     /lib/libnss_nis-2.7.so
7f1057725000-7f1057727000 rw-p 00009000 08:01 432578                     /lib/libnss_nis-2.7.so
7f1057727000-7f105773d000 r-xp 00000000 08:01 432562                     /lib/libnsl-2.7.so
7f105773d000-7f105793c000 ---p 00016000 08:01 432562                     /lib/libnsl-2.7.so
7f105793c000-7f105793e000 rw-p 00015000 08:01 432562                     /lib/libnsl-2.7.so
7f105793e000-7f1057940000 rw-p 7f105793e000 00:00 0 
7f1057940000-7f1057948000 r-xp 00000000 08:01 432564                     /lib/libnss_compat-2.7.so
7f1057948000-7f1057b47000 ---p 00008000 08:01 432564                     /lib/libnss_compat-2.7.so
7f1057b47000-7f1057b49000 rw-p 00007000 08:01 432564                     /lib/libnss_compat-2.7.so
7f1057b49000-7f1057b5f000 r-xp 00000000 08:01 432596                     /lib/libpthread-2.7.so
7f1057b5f000-7f1057d5f000 ---p 00016000 08:01 432596                     /lib/libpthread-2.7.so
7f1057d5f000-7f1057d61000 rw-p 00016000 08:01 432596                     /lib/libpthread-2.7.so
7f1057d61000-7f1057d65000 rw-p 7f1057d61000 00:00 0 
7f1057d65000-7f1057d6a000 r-xp 00000000 08:01 1103215                    /usr/lib/libXdmcp.so.6.0.0
7f1057d6a000-7f1057f69000 ---p 00005000 08:01 1103215                    /usr/lib/libXdmcp.so.6.0.0
7f1057f69000-7f1057f6a000 rw-p 00004000 08:01 1103215                    /usr/lib/libXdmcp.so.6.0.0
7f1057f6a000-7f1057f6c000 r-xp 00000000 08:01 1103204                    /usr/lib/libXau.so.6.0.0
7f1057f6c000-7f105816b000 ---p 00002000 08:01 1103204                    /usr/lib/libXau.so.6.0.0
7f105816b000-7f105816c000 rw-p 00001000 08:01 1103204                    /usr/lib/libXau.so.6.0.0
7f105816c000-7f105818e000 r-xp 00000000 08:01 1103441                    /usr/lib/libexpat.so.1.5.2
7f105818e000-7f105838e000 ---p 00022000 08:01 1103441                    /usr/lib/libexpat.so.1.5.2
7f105838e000-7f1058390000 rw-p 00022000 08:01 1103441                    /usr/lib/libexpat.so.1.5.2
7f1058390000-7f10583a1000 r-xp 00000000 08:01 1103219                    /usr/lib/libXext.so.6.4.0
7f10583a1000-7f10585a0000 ---p 00011000 08:01 1103219                    /usr/lib/libXext.so.6.4.0
7f10585a0000-7f10585a1000 rw-p 00010000 08:01 1103219                    /usr/lib/libXext.so.6.4.0
7f10585a1000-7f10585b7000 r-xp 00000000 08:01 1104055                    Traceback (most recent call last):
  File "./Calendar.py", line 14, in ?
    import adesklets
  File "/usr/lib/python2.4/site-packages/PIL/__init__.py", line 43, in ?
    
adesklets.error_handler.ADESKLETSError: adesklets process exited - 
Exception exceptions.AttributeError: <exceptions.AttributeError instance at 0x7ffb37afda70> in <bound method _Communicator.__del__ of <adesklets.communicator._Communicator instance at 0x7ffb36d07488>> ignored

Cheers, Tim.