Lord of the Rings Online, a MMORPG from Turbine apparently uses .NET. The system requirements explicitly say so. I am not sure what parts of the game are .NET and which are native code, but I assume that the launcher is written in .NET and I did spy some resource assemblies being downloaded in the update process. The version of .NET required is 1.1 with Service Pack 1. Which means on a Vista box framework 1.1 gets installed in the LOTRO setup.
Hmm need to explore more.
Anyway's here is 2 incriminating screens.
By the way, the public beta for the game is open.