[twitter-dev] Getting screen name from user id without rate limit

[Twixcel]

I'm trying to develop a twitter app that checks if someone is a
spammer. I have the userid, but to get the screenname I have to call
http://twitter.com/users/show.xml?id=<userid> which causes a limit
request.  Once I have the screen name I could get the data I need
without a limit request. Does anyone know a way to get the screen name
from a user Id without incurring a rate limit charge?