I updated the Waitsfield showroom Macs to 10.5.6 yesterday, and for some reason three of them decided it’d be best to go into an infinite restart loop instead of, oh, booting right up after the updates. It was bizarre that this exact problem occurred on three machines.
Because the problem occurred after running software updates, and on three machines, I knew this was a straightforward software problem that would likely be resolved with a simple archive and install from a Leopard installation disk. I inserted a standard Leopard install disk and expected the new Unibody MacBook Pro to boot right up. Instead, even while booting from the install disk the same infinite restart loop happened. I tried again, no luck.
I moved on to the Mac Pro, and the Leopard disk booted it right up. The Archive and Install fixed the problem. Same with the other victim, a MacBook Air. After this, I thought perhaps there was an actual hardware failure with the unibody MacBook Pro. I removed its hard drive, put it back together, and attempted to boot from the Leopard disk. No luck!
At this point, I was getting frustrated by this simple issue. I went to lunch, and pulling back into the parking lot it struck me: the Leopard disk contained 10.5.4, and the Unibody MacBook Pro requires 10.5.5 or later. Duh.
I found its system restore disk, the one that comes in the box, booted it right up and went ahead with the Archive and Install. A half hour later, it was back on display.