Mike Gleason Jr has a bit.ly client library written in C# called BitlyDotNet (it's hosted on here on Google Code). If it wasn't cool enough before, it just got more awesome.
I submitted a new version with the following improvements: 1. Added support for error code 208 ("you have exceeded your hourly rate limit for this method") 2. Overload to shorten multiple URLs in one shot 3. Fixed bug that occurred when hourly limit was exceeded (SingleOrDefault throws InvalidOperationException because of multiple errorCode nodes)
This library saved me time on a recent project and I'm glad I could contribute. Go check it out!