I wasn't sure where to vote in the Poll, since there was no "both" option.
My machine distribution works out like this:
Windows x86 (all XP Pro): 4 machines full time (plus two with dual-boot, but not used primarily)
Windows 7 x64: 2 (both set-up dual boot, but Windows 7 is the default)
Windows Vista x64: 1
MacOSX: 4 (one "triple-boots" to Windows 7 and XP)
Of those, I use my two licenses for MC heavily on 2 of the XP machines, the Vista 64 box, and on my Mac OSX/Windows 7/XP laptop. I also use it very occasionally (once per month or so) on one of the other Windows 7 boxes.
Also, FWIW, I keep all of the "Shell Integration" options in MC turned off, so I don't care if this is ever ported over to x64.
When Windows 7 is released, I'll be moving all but one of the XP boxes over to Windows 7 full time. I'll probably maintain one or two in a dual-boot configuration, with XP, but mostly for "just in case". I've had little problems with Windows 7 other than an occasional immature driver problem (mostly power-related problems on the Macbook Pro). I wouldn't use any 32-bit installation of Windows 7 or Vista. That's just a bunch of work for limited gain (I really wish they just wouldn't release dual versions, and just auto-install x64 on x64-capable hardware, more like OSX).