Last week I posted about upgrading RubyForge's installed Subversion version to 1.5. All seems well with that, and new repositories are being created as Subversion 1.5 repos, but that still leaves all the old repositories at 1.4. I don't want to yank the rug out from under anyone, so if you want your repo upgraded please file a support request. I've added a new category for that type of request, and I'll run svn-populate-node-origins-index on any converted repos so you should see decent performance.
Many thanks to Emiel van de Laar for filing a request and pointing me to the Subversion documentation on this!