27" iMac Hangs On Reboot I've got a 27" iMac 2.93Ghuz Intel Core i7 running 10.7.2. 

Anytime I reboot (like for some software updates), it gets stuck at the gray startup screen (no Apple logo). If I press and hold the power, turn it off, then turn it on, it boots as normal.

Any idea why a cold startup works but a warm reboot always hangs? Any idea how to diagnose it?
Try a verbose boot and see if the text output during the boot sequence provides any clues.