I'm having a rather strange problem with my dual 1.8GHz G5:
Whenever I turn it off (not just put it to sleep), it refuses to turn on again unless I either unplug the power cable from the back for a few seconds, or often I need to open up the case and remove the backup battery for a few seconds.
Powering up from sleep works as expected, it's only when it's been completely turned off that the problem occurs.
Any ideas what could be causing this problem? It's very annoying!!