I installed it on my 2011 21.5 imac this evening. I left the room while it was running the update and restarting. I came back a few minutes later and the screen was blank (I assumed the computer was off). I hit the power button and nothing happened, but I could hear the fans whirring, so I held the power button down to power down the computer, waited a few seconds, then turned it back on. I could hear the tone as it restarted, the CD drive did it's little dance, and it seemed like the computer was booted up, but still no screen.
At this point called Apple Support, who had me reset the PRAM, which didn't work. Then I booted up the computer, did a ctrl+eject+return restart, which didn't do anything. Then I turned the computer off, unplugged for 15-20 seconds, and restarted, again with no screen.
Now I have to take my day old iMac to a service center to have it looked at. And I just finished getting all my data transferred over from my old PC...alas.
Anyone have any tips?