A few days ago I wanted to install Win 7 on my 2010 iMac. After the install on first reboot on windows partition the screen was black. The problem is caused by the ati drivers that are included in windows. After some googling I have found an solution.
First of all you need the windows drivers for your machine. Start Boot Camp assistant in Application -> Utilities.
After you successfully download your drivers, copy the the WindowsSupport directory to an drive that is fat32 or ntfs formatted.
Now let's fix the black screen on Win 7 on iMac and Boot Camp.
After the installation of Win 7 boot your machine in Lion (hold alt key to change the boot partition), mount the windows partition and delete atikmdag.sys from c:/windows/system32/drivers.
To access your windows partition you need to install NTFS-3G and from Finder menu bar choose Go -> Go to Folder and type "/volumes".
Now you can see your boot camp partition and make changes to the files. Go to "/Volumes/BOOTCAMP/Windows/System32/drivers" and move to trash the "atikmdag.sys" file. Be sure to back up it first.
Now you can safely restart your machine in windows and finish the installation.
After you finish setting up your user account settings you can install your windows drivers.