I just tried using the SSH connection, and (where the URL has been a little bit anonymized just in case):

|> bzr branch svn+ssh://svn.concertant.com/XXXXX/YYYYY/ZZZZZ
bzr: ERROR: exceptions.AssertionError:                                                                                                                                         

Traceback (most recent call last):
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 896, in run_bzr_catch_errors
    return run_bzr(argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 842, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.5/site-packages/bzrlib/commands.py", line 542, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.5/site-packages/bzrlib/builtins.py", line 1045, in run
    source_branch=br_from)
  File "/home/users/russel/.bazaar/plugins/svn/remote.py", line 74, in sprout
    return super(SvnRemoteAccess, self).sprout(*args, **kwargs)
  File "/usr/lib/python2.5/site-packages/bzrlib/bzrdir.py", line 1116, in sprout
    result_repo.fetch(source_repository, revision_id=revision_id)
  File "/usr/lib/python2.5/site-packages/bzrlib/repository.py", line 1118, in fetch
    find_ghosts=find_ghosts)
  File "/home/users/russel/.bazaar/plugins/svn/fetch.py", line 1074, in fetch
    mapping, find_ghosts, pb=nested_pb)
  File "/home/users/russel/.bazaar/plugins/svn/fetch.py", line 922, in find_until
    self.find_mainline(foreign_revid, mapping, project, find_ghosts=find_ghosts, pb=pb)
  File "/home/users/russel/.bazaar/plugins/svn/fetch.py", line 905, in find_mainline
    foreign_revid, rhs_mapping = self.source.lookup_revision_id(p, project=project)
  File "/home/users/russel/.bazaar/plugins/svn/repository.py", line 832, in lookup_revision_id
    return self.revmap.get_branch_revnum(revid, layout, project)
  File "/home/users/russel/.bazaar/plugins/svn/revids.py", line 204, in get_branch_revnum
    for entry_revid, branch, revnum, mapping in self.actual.discover_revprop_revids(layout, last_revnum, last_checked, pb=pb):
  File "/home/users/russel/.bazaar/plugins/svn/revids.py", line 75, in discover_revprop_revids
    assert mapping is not None
AssertionError

bzr 1.12 on python 2.5.2 (linux2)
arguments: ['/usr/bin/bzr', 'branch', 'svn+ssh://svn.concertant.com/XXXXX/YYYYY/ZZZZZ']
encoding: 'UTF-8', fsenc: 'UTF-8', lang: 'en_GB.UTF-8'
plugins:
  bookmarks            /home/users/russel/.bazaar/plugins/bookmarks [unknown]
  bzrtools             /usr/lib/python2.5/site-packages/bzrlib/plugins/bzrtools [1.12]
  email                /usr/lib/python2.5/site-packages/bzrlib/plugins/email [unknown]
  gtk                  /home/users/russel/.bazaar/plugins/gtk [0.96.0.dev.1]
  launchpad            /usr/lib/python2.5/site-packages/bzrlib/plugins/launchpad [unknown]
  netrc_credential_store /usr/lib/python2.5/site-packages/bzrlib/plugins/netrc_credential_store [unknown]
  pqm                  /usr/lib/python2.5/site-packages/bzrlib/plugins/pqm [1.0dev]
  qbzr                 /home/users/russel/.bazaar/plugins/qbzr [0.9.9dev]
  rebase               /home/users/russel/.bazaar/plugins/rebase [0.4.4dev]
  stats                /usr/lib/python2.5/site-packages/bzrlib/plugins/stats [unknown]
  svn                  /home/users/russel/.bazaar/plugins/svn [0.5.3dev]
  upload               /usr/lib/python2.5/site-packages/bzrlib/plugins/upload [0.1]
  xmloutput            /home/users/russel/.bazaar/plugins/xmloutput [0.8.3]
*** 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.