Just finished installing Windows 7 on a new 27″ iMac, which was a long and painful process. Apple has stated that they will support Windows 7 via Boot Camp before the end of 2009, but an unsupported install is possible. People have reported problems with black screens, which is caused by a faulty ATI driver. Here’s how I pulled it off, using the information from this thread on MacRumors.

1. Partition your harddrive using Boot Camp. I’ve used a partition size of 50 GB, so I have enough to install Windows 7 (which takes ± 15 GB) and a modern game.
2. Insert the Windows 7 DVD and start the installation from Boot Camp. The Mac will restart and the installation process will commence.
3. After a certain point in the installation process — the part where it completes after a first reboot — the screen will go completely black. This is probably the moment when the display drivers are triggered. Do NOT abort the installation now by resetting the computer, the installation process continues despite the display problems.
4. Leave the computer in this mode for at least 20 minutes.
5. After that perform a hard reboot by powering down the computer and booting into the Windows partition.
6. You should now get the option to start in Safe Mode, choose this option.
7. The setup tries to continue in Safe Mode, but it can’t and you’ll get an error.
8. Do NOT click “Ok”. Instead press Shift F10 to get a recovery console.
9. Now type compmgmt.msc to bring up the Computer Management Screen.
10. Select the Device Manager and disable the video card under Display Adapters. Do NOT uninstall it, because you’ll end up in all kinds of hell.
11. Close the Device Manager and click “Ok” on the aforementioned error message (7).
12. This will trigger another reboot, but this time you should have video, albeit at a crippled resolution.
13. Let the installation continue and log in to your new Windows 7 installation.
14. Insert the disc with the Boot Camp Drivers (which is your Mac OS X installation disc). Do NOT choose to let Autorun.exe do its magic.
15. Instead, enter cmd in the search field of the Windows Start Menu.
16. Before clicking the entry, right-click on it and select Run as administrator from the context menu. After that, click the item to fire up cmd.exe
17. Enter the following commands:
    

    D:
cd "Boot Camp\Drivers\Apple"
msiexec /i BootCamp64.msi

    Which will fire up the 64-bit version of installation of the Boot Camp drivers.

18. Restart your machine after the install and you will notice the screen resolution still isn’t right. You can choose a lot more options, but the widescreen variety isn’t listed.
19. Fire up Windows Update from the Start Menu and let it check for updates.
20. There should be an item under Optional updates referring to your ATI graphics card, which is either a ATI Radeon HD 4670 or HD 4850.
21. Install this update and reboot when asked. Windows 7 should reboot in the iMac’s native resolution and your done!

It’s not a guaranteed success, so your mileage may vary. If your first try doesn’t work out, consider trying again.