How to detect programmatically whether you are running on 64-bit Windows

Something that came up in IM a few minutes ago: How to detect programmatically whether you are running on 64-bit Windows.

This is going to become increasingly more important as people migrate from 32-bit processors to 64-bit processors.

I'm a little torn on this one, since .NET Framework APIs exist for asking if the CPU is 64-bits or not. But for the most comprehensively portable solution, you have to ignore .NET and find something that will still run on, say, Windows 95.