FWIW, this doesn't seem to have fixed my problem. I cobbled together and installed a dulwich package to try this out (lp:~jelmer/dulwich/debian r199, with r226 of http://people.samba.org/bzr/jelmer/dulwich/trunk/ applied on top of this; also lp:~bzr/bzr-git/trunk r340 since that seems to be the most recent version that works with the version of bzr I have). I also made a change similar to Ivan's above to print out the parameters passed to mmap.mmap(). The result was as follows:

$ bzr get git://git.kernel.org/pub/scm/linux/hotplug/udev.git
/usr/lib/python2.6/dist-packages/dulwich/objects.py:25: DeprecationWarning: the sha module is deprecated; use the hashlib module instead
  import sha
6 4517536 1 0        ] git: Total 24442 (delta 13944), reused 24183 (delta 13760) 0
6 4056275 1 112
bzr: ERROR: mmap.error: [Errno 22] Invalid argument

Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 716, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 911, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.6/dist-packages/bzrlib/commands.py", line 547, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.6/dist-packages/bzrlib/builtins.py", line 1100, in run
    source_branch=br_from)
  File "/usr/lib/python2.6/dist-packages/bzrlib/bzrdir.py", line 1134, in sprout
    result_repo.fetch(source_repository, fetch_spec=fetch_spec)
  File "/usr/lib/python2.6/dist-packages/bzrlib/repository.py", line 1133, in fetch
    find_ghosts=find_ghosts, fetch_spec=fetch_spec)
  File "/home/cjwatson/.bazaar/plugins/git/fetch.py", line 353, in fetch
    mapping=mapping, fetch_spec=fetch_spec)
  File "/home/cjwatson/.bazaar/plugins/git/fetch.py", line 373, in fetch_refs
    self.fetch_objects(determine_wants, mapping, pb)
  File "/home/cjwatson/.bazaar/plugins/git/fetch.py", line 408, in fetch_objects
    target_git_object_retriever, recorded_wants, pb)
  File "/home/cjwatson/.bazaar/plugins/git/fetch.py", line 278, in import_git_objects
    o = object_iter[head]
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 953, in __getitem__
    type, uncomp = self.get_raw(sha1)
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 943, in get_raw
    offset = self.idx.object_index(sha1)
  File "/home/cjwatson/.bazaar/plugins/git/remote.py", line 192, in idx
    self._data.create_index_v2(self._idx_path, self.resolve_ext_ref)
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 551, in create_index_v2
    entries = self.sorted_entries(resolve_ext_ref)
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 542, in sorted_entries
    ret = list(self.iterentries(resolve_ext_ref))
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 529, in iterentries
    type, obj = self.resolve_object(offset, type, obj, get_ref_text)
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 480, in resolve_object
    type, base_obj = get_offset(base_offset)
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 572, in get_object_at
    map, map_offset = simple_mmap(self._file, offset, self._size-offset)
  File "/usr/lib/python2.6/dist-packages/dulwich/pack.py", line 113, in simple_mmap
    offset=int(offset / mmap.ALLOCATIONGRANULARITY)),
error: [Errno 22] Invalid argument

bzr 1.13.1 on python 2.6.2 (linux2)
arguments: ['/usr/bin/bzr', 'get', 'git://git.kernel.org/pub/scm/linux/hotplug/udev.git']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_GB.UTF-8'
plugins:
  bazimport            /home/cjwatson/.bazaar/plugins/bazimport [1.6]
  builddeb             /usr/lib/python2.6/dist-packages/bzrlib/plugins/builddeb [unknown]
  bzrtools             /usr/lib/python2.6/dist-packages/bzrlib/plugins/bzrtools [1.13]
  cia                  /usr/lib/python2.6/dist-packages/bzrlib/plugins/cia [1.0dev]
  cvsps_import         /usr/lib/python2.6/dist-packages/bzrlib/plugins/cvsps_import [unknown]
  fastimport           /home/cjwatson/.bazaar/plugins/fastimport [unknown]
  git                  /home/cjwatson/.bazaar/plugins/git [0.2dev]
  gtk                  /usr/lib/python2.6/dist-packages/bzrlib/plugins/gtk [0.96.0.dev.1]
  launchpad            /usr/lib/python2.6/dist-packages/bzrlib/plugins/launchpad [unknown]
  netrc_credential_store /usr/lib/python2.6/dist-packages/bzrlib/plugins/netrc_credential_store [unknown]
  push_and_update      /home/cjwatson/.bazaar/plugins/push_and_update [0.2dev]
  push_fix             /home/cjwatson/.bazaar/plugins/push_fix [unknown]
  rebase               /usr/lib/python2.6/dist-packages/bzrlib/plugins/rebase [0.4.4]
  vimdiff              /home/cjwatson/.bazaar/plugins/vimdiff [unknown]
*** Bazaar has encountered an internal error.
    Please report a bug at https://bugs.launchpad.net/bzr/+filebug
    including this traceback, and a description of what you
    were doing when the error occurred.