Based on nothing more than my own messing around with my (also 2011) iMac and my Airport Extreme, my personal suspicion is that the issue is that the computer, for whatever reason, does not maintain its registration with the Bonjour sleep proxy, rather than the machine not being able to take delivery and wake up to WOL packets.
My suspicion regarding this suspicion is that the iMac's problems with maintaining its registration with the Bonjour sleep proxy is caused by a bug in the SMC.
If anybody is interested in how I've arrived at this position then I will take the time to outline my observations in the hope that somebody more intelligent than I will be able to take things further (or at least discredit them as stupid), but I personally cannot be bothered, for the time being, to try and investigate this any further. My plan is to wait and see if Mountain Lion (or even 10.7.4) fixes the issue, and then carefully consider my next move from there if it doesn't.
Irrespective, shelling out for ML in the hope that it will fix something that should have worked from day one will be a bitter pill to swallow.