I had lunch with my brother-in-law Eriks on Sunday. He recently bought a new 24-inch aluminum iMac (described by Apple as Late 2007"). The machine comes with Tiger pre-installed and he went ahead and upgraded the machine to Leopard.
Unhappily, he reported that after installing Leopard, he restarted the machine, entered his password in the login window. The machine appeared to accept it (if you enter a wrong password, the screen "shakes") and the screen turned blue as if it was about to load the desktop. But then the login window would reappear.
And this on a brand new iMac out of the box.
It appears he's not alone. Apple put out a technical note on the subject today: Mac OS X 10.5, iMac (Late 2007): Stuck in a "loop" at the login window after upgrading to Leopard.
The note suggests that users start up from the Leopard Install DVD (you will need to hold down the C key when restarting) and then perform an Archive and Install reinstallation (while being sure to select Preserve Users and Network Settings).
However, a guru at his local Apple Store suggested that he do the more drastic Erase and Install option, which is what he did. That fixed the problem, Eriks said.
After the new Leopard installation is complete, the Apple support doc suggests that users install the iMac Software Update 1.3 for Leopard. I asked Eriks to check whether Software Update flagged this update but it didn't; so be sure to install the update if you have a new 20-inch and 24-inch Late 2007 model iMac. The note said the problem machines have either a 2GHZ, 2.4GHz and 2.8GHz processor.
For more posts on the Leopard Update, check out: