Did you install this update by itself, or were there other updates installed at the same time? Is the iMac a new model?
I would hold the power button down for 5-10 seconds to shut it down, then try restarting and see what happens. If you get it restarted, run repair disk and repair permissions. If it's a new iMac, you've probably downloaded a lot of updates, and it could be this. Good luck!
Morod
I had a similar problem on my macbook. After Software Updates had finished downloading the package and I hit OK when asked to reboot, the machine wouldn't reboot because it allegedly couldn't terminate the apps that I had running (although Process Monitor revealed that they weren't running anymore). I force rebooted from the console (sudo shutdown -r now).
After rebooting, I ran Software Updates again, and the installation proceeded without problems.