I keep my 3.06 Ghz 24" alum iMac on 24/7. I left my office last night around 11p with it configured to go to sleep mode after 30 mins (like normal). Haven't had any changes to that setting in several months. I stepped into my office this morning to find that my standard 'press of the enter key' did not wake it from sleep. i then hit the power button on the back of the unit... nothing. pressed and held it..... nothing. again.... same story. i checked the UPC, and it was powered up. the other peripherals connected to the UPC were operating just as expected, so it wasn't a power issue. i unplugged the iMac for a few mins, then plugged it back in. Pressed the button on the back, and viola, it booted up like normal - but displayed an error box that stated an unrecoverable OSX error had been encountered. I clicked OK, and now all appears to be running fine. The first thing i did was check out the OSX version number to see if it was was the result of an update. 10.5.6 was displayed in the version info. Is this an mere coincidence that this occurred over the evening that 10.5.7 was released, or could it have been attributed to an attempted upgrade? I did a scan of the forums, and haven't seen any similar problems reported recently.