Someone pointed me to the following page today: Features unavailable on 64-bit versions of the Windows .NET Server 2003 family. I was surprised to discover that .NET is not supported on the 64-bit version. Some research uncovered this reply from Microsoftie Larry Sullivan here:
The 64bit .Net Framework is under development for release in a future version. The IL is not changed to support 64bit and your managed applications in most instances will run as native 64bit processes. There are some constructs to watch for that can cause some cross platform bugs. These include explicit layout of structures, PInvoke and COM Interop to dll's or COM components where no 64bit equivilent exists. When doing PInvoke use IntPtr instead of int for HANDLES and pointers. Thats the set of issues where you can get into trouble otherwise managed code runs unchanged.
A surprising omission from something called .NET Server.