Eventually the checkout finished.  On irc jelmer suggested that it was slow because 2.4 copies more revisions than earlier versions of bzr did.

Shortly after it finished, this happened:

bzr: failed to report crash using apport:
     OSError(17, 'File exists')
bzr: ERROR: exceptions.AssertionError: no lhs parent revmeta found for <bzrlib.plugins.svn.revmeta.CachingBzrMetaRevision object at 0x714aa70>

Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 946, in exception_to_return_code
    return the_callable(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 1150, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 699, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 721, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 135, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 165, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/builtins.py", line 1408, in run
    accelerator_tree, hardlink)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/branch.py", line 375, in create_checkout
    hardlink=hardlink)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/branch.py", line 320, in _create_heavyweight_checkout
    checkout_branch.pull(self, stop_revision=revision_id)
  File "/usr/lib/python2.7/dist-packages/bzrlib/branch.py", line 1108, in pull
    possible_transports=possible_transports, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/branch.py", line 802, in pull
    stop_revision, overwrite)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/branch.py", line 734, in _update_revisions
    self.fetch(stop_revision=stop_revision, fetch_tags=fetch_tags)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/branch.py", line 705, in fetch
    d = resolve_tags_svn_ancestry(self.source, tag_revmetas)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/tags.py", line 94, in resolve_tags_svn_ancestry
    pb=pb):
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/repository.py", line 845, in _iter_reverse_revmeta_mapping_ancestry
    rhs_parent_foreign_revid, rhs_parent_mapping = self.lookup_bzr_revision_id(rhs_parent_revid, foreign_sibling=revmeta.metarev.get_foreign_revid())
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/repository.py", line 1064, in lookup_bzr_revision_id
    return self.revmap.get_branch_revnum(revid, layout, project)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/revids.py", line 290, in get_branch_revnum
    branch_path, min_revnum, max_revnum)
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/revids.py", line 151, in bisect_fileprop_revid_revnum
    for propname, (oldpropvalue, propvalue) in revmeta.get_changed_fileprops().iteritems():
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/revmeta.py", line 224, in get_changed_fileprops
    self.get_fileprops(), self.get_previous_fileprops())
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/revmeta.py", line 217, in get_previous_fileprops
    return prev.get_fileprops()
  File "/usr/lib/python2.7/dist-packages/bzrlib/plugins/svn/revmeta.py", line 187, in get_fileprops
    "no lhs parent revmeta found for %r" % lm
AssertionError: no lhs parent revmeta found for <bzrlib.plugins.svn.revmeta.CachingBzrMetaRevision object at 0x714aa70>

bzr 2.4b5 on python 2.7.1 (Linux-2.6.38-10-generic-x86_64-with-
    Ubuntu-11.04-natty)
arguments: ['/usr/bin/bzr', 'checkout',
    'svn://svn.twistedmatrix.com/svn/Twisted//branches/listopenfds-resource-
    fallback-5052-2', 'Twisted']
plugins: bash_completion[2.4b5], bzrtools[2.4.0], changelog_merge[2.4b5],
    launchpad[2.4b5], netrc_credential_store[2.4b5], news_merge[2.4b5],
    svn[1.1.0dev], weave_fmt[2.4b5]
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_US.UTF-8'

*** Bazaar has encountered an internal error.  This probably indicates a
    bug in Bazaar.  You can help us fix it by filing a bug report at
        https://bugs.launchpad.net/bzr/+filebug
    including this traceback and a description of the problem.
elapsedTime=3.532380
program finished with exit code 4

Whether it is the same bug but now manifesting with a new symptom or a new bug, I don't know.