Why won't you just sleep!

My G5 has a problem, it won’t sleep.

Since I’ve had a Mac I always sleep the machine rather than power off, not sure why but it just sounded like a good plan. It does not take so long to start up again so that’s a plus in my book!
But for the last couple of days now it has been unwilling to just let go and put it’s head down. You select the sleep option and the screen blacks, you hear the noise of the fans slowing, then the hard drive’s stopping and that final click to say I’m gone. But now straight after that click I get a second click; and low and behold the machine wakes up again.
I’ve tried removing all devices so it’s not a USB/Firewire problem, I’ve repaired permissions, updated cleaned out and even tried Nightol but nothing works. Looking through the logs it would appear to be a network related problem, so I try DHCP config and even a manual, still no real progress. When in DHCP mode I can get it to sleep after about the 4th time but it’s a bit like a toddler that won’t go to bed on time!