Boston uses a dropdown to allow you to select from existing categories/locales. I do this via a CGI::Wiki::Plugin module (attached: summariser.pm) and some changes to wiki.cgi (also attached, although that patch has some cruft.) The categories are then iterated in the edit_form.tt template to create a dropdown box with some javascript.
I'm not sure what the best way to do this is, but I'd like to see it pulled into the core, because more than one person has asked for it, and it'll be one less thing for me to maintain on my own.
What's a better way to do this? Anyone have any suggestions?