I have a Open source C# caching providers for a number of clients including Memcached at:
http://code.google.com/p/servicestack/wiki/Caching
The memcached client which we've been using in production for over a year uses enyim's memcached client underneath it's BSD licenced so feel free to use it, source available here.
I should also mention that Marc Gravel (2nd on StackOverflow) has embedded protobuf-net with enyim's memcache client as well, details available on his blog:
http://marcgravell.blogspot.com/2010/01/distributed-caching-with-protobuf-net.html