Seeing all the UNIX guys have memcache (PHP, Python, etc.), are the .NET libraries stress tested and equally efficient? (or more?)
Consider Microsoft's Velocity cache. It'll probably be the one that gets the largest support and stress testing.
Velocity is still in Cummunity technical preview (CTP). But I would agree that it's the right technology for people to evaluate from Microsoft for this purpose.
Also, just so it's considered - if you're already using memcached in your environement there is a client library for C# - memcacheddotnet.
I used the memcached client a while back (before rejoining Microsoft) only for testing and not in production - it held up great under a simulated load of my system for 50+ concurrent user testing (which met my requirements) - never went to produciton with it though (timing didn't work out and wasn't strictly needed to satisfy anything business critical at the time).