I have a first release of my OAuth implementation up on google code, you can read a little more about it here and the code can be found here. It's released under the MIT license, basically do what you like with it.
The library includes both consumer and a provider implementations - test coverage is pretty poor, as it was originally put together as part of my REST presentation to the local Ellerslie .Net user group a few weeks ago, but as I work through the code and rewrite various sections in my spare time it should become a little more
I have not added the MonoRail examples back to the code yet - but will shortly - and will probably provide a WCF implementation as well.
I've also posted a short intro to OAuth - OAuth for beginners - for those who are interested.