I promote 64-bit a lot because I personally think my 64-bit machine is snappier and more stable. I also have buttloads of RAM which is nice.
The first question developer ask me when I'm pushing 64-bit is "Can I still run Visual Studio the same? Can I make apps that run everywhere?" Yes, totally. I run VS2008 all day on x64 machines writing ASP.NET apps, WPF apps, and Console apps and don't give it a thought. When I'm developing I usually don't sweat any of this. Visual Studio 2008 installs and runs just fine.
If you care about details, when you install .NET on a 64-bit machine the package is bigger because you're getting BOTH 32-bit and 64-bit versions of stuff. Some of the things that are 64-bit specific are: