I purchased a new 24" iMac three weeks ago, and loaded Vista 64 on it. Here's what I learned in the process:
You need the 64-bit drivers from the Mac Pro. You can get them from the website listed in a previous note. My daughter has a Macbook Air, and I started with those, but it didn't have the full set of drivers needed for the iMac. Since it is a laptop, it didn't have (for example) the driver for the aluminum keyboard. The Mac Pro disc has everything that is needed.
You'll need to start with a 64-bit OS, not a 32-bit OS. For whatever reason, if a 32-bit OS is loaded first (e.g., XP 32, or Vista 32), you can't get the upgrade to a 64-bit OS to work with Boot Camp. In my case, I started with XP-64 (full install) and then used a Vista 64 upgrade disk.
The process I used was:
1. Run Boot Camp and load XP-64 first.
2. Install Vista-64 upgrade.
3. Load the individual 64-bit drivers from the Mac Pro disc.
4. Finally, run the latest Boot Camp upgrade for Vista 64 (available on the Apple website.
I've had no problems at all with Vista on the iMac. We only use Vista to run games (Microsoft Flight Simulator, Crysis, Oblivion). Our iMac is a 3.06 with the nVidia 8800 GS video card, and it runs those games very well.
By the way, if you do gaming on the iMac, be aware that the nVidia card is seriously underclocked. You can boost the clock speed using a utility called "Rivatuner" and get much better performance. No doubt this increases the temperature of the nVidia card, but we haven't had any problems.