Using additional fields in search

[ZiNgA BuRgA]

You can't really "search" however you can filter.
For an example, try clicking on the thread tags here: http://mybbhacks.zingaburga.com/forumdisplay.php?fid=25

Note that XThreads isn't particularly great with filtering (or searching if that feature ever gets added) on multivalued fields, such as thread tags.  Although it "works", it would probably be better to have a dedicated plugin for this, if you have quite a number of threads to search through.