Looks like there might be some changes in the xapian bindings that round is using by default if they exist.  On a clean install on CentOS5, running python 2.4.3 with the stock python and xapian-bindings packages (and latest roundup) I'm getting an error when trying to create tickets:

In  /usr/lib/python2.4/site-packages/roundup/backends/indexer_xapian.py

Line 95:

           < term = stemmer.stem_word(word)
           > term = stemmer(word)

Info about the xapian binding changes:

http://lists.tartarus.org/pipermail/xapian-commits/2007-April/003162.html

And the complete stacktrace:

Traceback (most recent call last):

  File "/usr/lib/python2.4/site-packages/roundup/cgi/client.py", line 265, in inner_main
    html = self.handle_action()

  File "/usr/lib/python2.4/site-packages/roundup/cgi/client.py", line 820, in handle_action
    return action_klass(self).execute()

  File "/usr/lib/python2.4/site-packages/roundup/cgi/actions.py", line 39, in execute
    return self.handle()

  File "/usr/lib/python2.4/site-packages/roundup/cgi/actions.py", line 616, in handle
    messages = self._editnodes(props, links)

  File "/usr/lib/python2.4/site-packages/roundup/cgi/actions.py", line 426, in _editnodes
    newid = self._createnode(cn, props)

  File "/usr/lib/python2.4/site-packages/roundup/cgi/actions.py", line 481, in _createnode
    return cl.create(**props)

  File "/usr/lib/python2.4/site-packages/roundup/backends/rdbms_common.py", line 2603, in create
    content, mime_type)

  File "/usr/lib/python2.4/site-packages/roundup/backends/indexer_xapian.py", line 95, in add_text
    term = stemmer.stem_word(word)

AttributeError: 'Stem' object has no attribute 'stem_word'

Making the listed change seems to fix everything right up and I'm able to create tickets now without a problem.