I've been having this problem for a while now on my 1Ghz 17" iMac. Sometimes, when the computer has been idle long enough to go back to the login screen, it won't let me get back in to my account properly. My user account is technically logged in, but when I click on the name I get the cube-rotating-eye-candy thing over to my desktop for a split second, and then it rotates right back to the login screen. I have to reboot to get back into my account. It's a bit of a pain..... anyone else had this? Or have any ideas on what to do? I don't want to disable having it automatically go back to the login screen, as sometimes my gf uses the computer.....and she really doesn't need to know what's going on in my account 
Help would be appreciated!
Help would be appreciated!