Sleep-wake

Any idea why it might wake/fail to sleep?
Rob wrote on :

My iMac (C2D latest OS etc) seems to wake at random and then fail to sleep after the period set in Preferences. The wake event is recorded as:

Oct 22 21:06:42 upstairs-2 configd[14]: AppleTalk shutdown Oct 22 21:06:43 upstairs-2 kernel[0]: System Sleep Oct 22 21:06:42 upstairs-2 configd[14]: AppleTalk shutdown complete Oct 23 04:40:01 upstairs-2 kernel[0]: System Wake Oct 23 04:39:53 upstairs-2 airportd[3855]: FastFinder scanning channels again

The computer will stay switched on with this system.log message every two minutes or so:

Oct 23 10:00:33 upstairs-2 ntpd[27]: sendto(17.72.255.12) (fd=23): No route to host

Any idea why it might wake/fail to sleep?

Thanks, Rob