.

If you buy the 64-bit edition, it is.

How is the driver situation?

Good enough to run enough of some top-end CAD. Video is ok if you hve the right card but printing can suck.

....ohhhh wait, I was thinking of XP 64-bit.

As long as you stick to recent, mainstream hardware, you've got a decent chance of getting 64-bit drivers. 

Check the mfr's website before you buy.

I've got a coworker running 64 bit Vista and he loves it more than he liked XP (and he was a VERY late adopter on his home machine)

He's a pretty hardcore gamer, too.

I'm running 64bit Vista as my main OS (and have been since the Nov launch)

Driver situation is a pretty good on my laptop (bought April 2006), I'm still using the "inbox" video driver for the nVidia 7800 which I had to force select but I don't game on this machine (and nVidia are useless at providing mobile drivers). The only driver I can't find is the webcam built into the screen frame. Some of the other drivers I'm running are XP64 drivers.

Your milage will vary depending on peripherals.

The only annoyance is that Visual studio 2005 doesn't support changing code while debugging (which 2003 and earlier didn't anyway).

Oh printing hasn't sucked yet for me (Mostly HP lasers + a couple of high end colour lasers), but I think some of the bottom end printers won't ever get drivers.

"He's a pretty hardcore gamer, too."

I usually defend Vista, but 64-bit and gaming is dicey. It is less the games and OS than it is the drivers to run that nice video/sound card combo. If you play older games, be wary, or plan to dual boot XP.

Also, there is no IPX support. If you want multiplayer in many older titles, it is another argument to dual boot.