All good answers, LinqToTwitter good. Also check out my post explaining the basics of using the Twitter API from C#/LINQ, including being aware of rate limits. (Which is important to understand).
http://stuff.seans.com/2009/04/04/a-simple-net-twitter-api-wrapper-using-linq/
Coming soon - a version of my code that automatically adjusts request speed to your rate limit. (Which is either 100/hr by default, or 20,000/hr if you or your site is "white listed").