It was initially released for only the Windows platform. The Microsoft provided full framework is still targeted only at Windows. I say "full" because there are now some variations that run elsewhere. Silverlight is a subset of the .NET 3.0 framework and it targeted to run on several prominent OSes (Windows, Mac, Linux).

There is an open source project, the Mono Project which promises to run .NET on many different platforms.

-Peter
compiledthoughts.com