Hey, I'm one of the guys who built WhoShouldiFollow. Glad the recommendations are working out for you.

The Twitter API rate-limiting makes things harder, but as a whitelisted app we haven't (yet) run into any situations where we've hit our quota or get throttled in any way.

It sounds like Alex is working on a new method to retrieve a users followers/friends in bulk which should help out a lot: http://groups.google.com/group/twitter-development-talk/msg/...