I'm using Win7 X64 with no problem in Bootcamp. I installed Win7, did the online updates and almost everything worked out of the box. I then put in the SL DVD and ran "setup.exe" and let it do it's thing. Everything loaded up just fine including the Bootcamp Control Panel and screen brightness adjustment seems to work fine from there.
I have a new 27" dual core and I can tell you two things to watch for. 1. using the SL disk drivers it loaded up Cirrus Logic drivers for the audio. It "worked" but everytime I booted it showed the audio as not installed. You could tell it to reload the drivers and then it worked fine until the next re-boot. I simply rolled back to the original high def Win7 drivers that were installed by default and they work flawlessly so watch for that.
2. Video drivers. Win7 installed some generic ATI drivers for my 4670 card so I downloaded the 9.10 drivers from ATI and tried to install those. No go. No Catalyst Control Panel and no drivers updated. It did however create an ATI directory on the C: drive which you're going to need. Go to this website:
http://www.driverheaven.net/modtool.php and download the tool.
Follow the Vista instructions further down on the main page and then the ATI drivers will install. You don't need to disable UAC though as they recommend in the Vista instructions as UAC in Win7 is a lot smarter now. Once it modified my drivers in the ATI directory on the C: drive I ran "setup" from that directory and Voila! Working 9.10 drivers including Catalyst and it was all super simple and fully automatic with Modtool. Very slick.
I guess the onboard 4670 is a mobile version of the GPU so that's why Modtool works on it. Anyhoo, hope that helps and good luck! Cheers!
James
P.S. Please note: These instructions are for the new fall 2009 dual core iMac's so if you're using anything else, then this may not apply or work for you.