iMac reachability over sleep
I found someone else with this specific problem.  My understanding is that Apple's Airport Base Stations simply "forget" the previously attached computer.  

Two things to check:


Create a static IP address in network configuration on the iMac
in Airport Utility make sure you've created a MAC address based DHCP reservation for this iMac.  Apparently, the AEX AEB &amp; TC will still "forget" but it might be longer now, like 6-8 hours.  


The work around: using either and application or System Preferences > Energy Save schedule the iMac to "wake up" and turn back off every 6 hours or so if not being used.  This should re-up the IP.  Alternatively, DD-WRT router software supports WOL out of the box.  Wireless WOL is not part of the spec, however, I believe wireless WOL is an Apple deal.