THANK GOODNESS... and glatzsa! This worked for my iMac27" late-2009 as well.
Power down the computer, Press and hold Shift + Control + Option simultaneously, press and release the power button and release Shift + Control + Option (after I heard the chime) and my system came right up.
The Long Story:
My iMac would not boot up, it would get stuck on the grey (or white) screen after the Apple and Progress Bar screen.
Three days of Safe Boot, Recovery Mode, Disk Utility checks/repairs, Single User Mode, resetting NVRAM (formerly known as PRAM), and IM'ing with two different Apple Geniuses (they were very cool, BTW).... produced no results. Finally I had to wipe my HDD and reinstall. THEN.... I got stuck on the "Setting Up Your Mac" page just like the original post, for about two-plus hours. By then I was at my wit's end.
So thanks again! ๐
BTW, just to make sure, I just ran a Restart and everything booted up just fine!