After many boots and troubleshooting of a startup problem it appears that it is due to the USB connection on my older cinema display. If I leave the USB cable from the display unplugged it boots fine. If I plug it in, the mini stays stuck on the grey screen with no apple, no spinning wheel. Seems that this may have begun after a 10.5.6 upgrade, so may need to post there, but just wanted to check compatibility. I dont use the USB ports on the back of the display often, but always nice to know they are there in a pinch.