Apple has issued a small firmware update to resolve graphics glitches on 2011 iMacs running Mac OS X 10.6.7. These issues manifest themselves as a white screen during startup, causing your computer to become unresponsive, or vertical lines on the screen while in use. From the release notes:
This firmware update fixes an issue that in rare cases may cause an iMac to hang during startup or waking from sleep. Do not shut off the power on your iMac during this update. Loss of power could result in your iMac failing to start up.
You can either grab the standalone iMac Graphic FW Update 2.0 (a 700KB download) if your mid-2011 iMac exhibits the aforementioned issues or choose Software Update from the Apple menu and follow the onscreen instructions.