Twitter has already launched funtionality that allows you to search profiles and find people of related interests, or whom are near you. But more than that is possible. Twitter is actively working on adding text based search as well. Here is what Jack had to say about it:
Another big aspect is text-based search. That's next, where you will be able to search through any of our updates and search for activities like search for who is reading right now, or who is walking right now, or who is eating right now, who is in San Francisco right now, etc. It really opens up some very interesting possibilities of how this system benefits you, what you can do with it, when you know that five people in your general radius are eating Mexican food right now. It's a different world.
The idea gives a whole new meaning to the concept of "site search". Now we are talking about a custom search application designed to process a dynamic data stream in real time.