Twitter's own search facilities aren't terrible, but don't offer a lot in the way of filtering. Aussie Twitter Search features a very specific type of fiktering: type in a keyword and it will list Australians who have that word as part of their biography.
The service isn't perfect: a search for Lifehacker, for instance, didn't find me. However, if you're trying to identify locals interested in a particular topic, it has potential. You can optionally download a full listing of the bios for any result set.