From the folks that brought you DotNetOpenId...
This library (will) bring a full implementation of the OAuth protocol, written in C#.
It's in alpha form right now, due to some impending refactoring that needs to be done to improve usability, but it's quite fully functional now. It's got smart client consumer, web client consumer, web service provider and WCF samples.
These details are provided for information only. No information here is legal advice and should not be used as such.