I am having the same problem. After running and installing App Store software update an error message appeared after about 7 minutes saying the installation failed and could not continue. A restart came to to login screen. Adding my account password spun the circle a few moments then the screen flashed and no login. I tried again. Tried the guest account. Tried zapping PRAM. Tried recovery partition to repair disk permissions. I was able to get an error message to get a new hard drive from a failed disk repair.
2.5 Ghz MacBook Pro 15" A1260 titanium pre-unibody.
Tried safe mode. Get only about 30 seconds of spinning, 1/4 on the progress bar, and then shut down.
I doubt it is a bad disk. I say that because my Windows partition still boots normally using Option at startup.
I read several forums and posts. Found something about a possible glitch with video card switching permissions. Cannot get into any account to change permissions, so I don't know if that is possible at this point.
Found something to startup in single user Command + S mode and check disk persmissions. Something about the 8th character ending in "r" seemed to be correct. A /fsck -fy resulting in an error at catalog, if I recall. A bad hard drive could be possible, but seems really unlikely as I have had no recent disk issues and the Bootcamp partition works fine.
There are a growing number of similar posts on no reboot after update or no login, etc. I doubt sending people out for new drives or kernel panics is the issue. Something with the install failed and is preventing reboot. Any help would be appreciated. I'm on a different computer as I cannot startup in any manner.
I am hopeing it is a problem with the new update and not something to do with the harddrive! What worries me is the code that shows over the white boot up screen! and if i try and boot up in "Safe Mode" i sometimes get the white screen and 3 beeps! i have read that these beeps could be to do with RAM!