Are there any real MMOs where the server engine is based on .Net?
Photon Socket Server is a middleware provider which is using C++ as the underlying framework but the bulk of the logic (i.e. what you have access to) is using .NET.
Lord of the Rings Online's client uses .NET 1.1. Not sure about the server. I think Turbine's games (LotRO, DDO, Asheron's Call) use .NET.
Vindictus server is fully written in .Net. Vindictus offers only one logical shard with hundreds of thousands of simultaneous users, which can be done via function and data distributed computing techniques.