Working, for now:
I've posted on this thread a few times. I was having the problem for a while. At one point, two or three months ago, I did a clean install of my iMac with Mountain Lion, can't remember what version. It worked for a while. And then stopped. Then it wasn't going to sleep on schedule either.
I applied the Combo update for 10.8.3 and my Mac now goes to sleep as configured and wakes up when I use an ssh app called ServerAuditor (free). Kind of a weird name but a pretty nice SSH app. I just looked in my Console and it shows a magic packet was received when I connected with the app. If you're willing to enable Remote Login in Sharing you could try it as a wakeup method.
3/20/13 8:46:48.000 AM kernel[0]: Ethernet [AppleBCM5701Ethernet]: Wake reason GIGE - magic packet received
I have to admit, I tried just about everything that's been posted on this thread. So your results will no doubt vary. But the combo updater might be worth a try if you haven't gone that route yet.
I have a 27" i7 iMac that's plugged into my network via ethernet cable.