On another forum, there seems to be a consensus that the Calendar app is preventing many phones from truly going to sleep mode. You can check to see if your Awake Time = Up Time. If it is, it's likely that your Calendar is not allowing your phone to go to sleep - it's still Awake, even when the screen is black, and is killing your battery.
To check go to Menu > Settings > About Phone > Battery
If the two stats are the same, try this: Go to Menu > Settings > Manage Apps and look for the Calendar and do a Force Close. You can relaunch the Calendar and the phone will start to go to sleep.
It's only when the Calendar is allowed to stay open right after the phone has shut down and re-started that the phone won't truly go to sleep mode.
I'm just trying this today and already my Up Time and Awake time are dramatically different. I'm also seeing somewhat improved battery performance.