Such a dramatic battery drain suggests that the phone isn't sleeping, like Darkschneider suggested. Compare your up time to your awake time (Settings->About phone->Status, scroll to bottom) and make sure you're not sitting at a high percentage awake time.
Another thing that could be a drain is if you've got your phone set to high performance instead of ondemand. If you use setCPU or gbhil's script to set your phone to max processing speed, try reducing it to the ondemand setting. (By the way, I wouldn't use both, as I tried that and it slowed my phone and started draining my battery faster.) If none of that looks familiar to you, don't worry about it; gbhil has the setting defaulted to ondemand.
Other settings that might help increase battery life:
-Turn off always on mobile (Settings->Wireless controls->Mobile Network Settings)
-Make sure that GPS and WiFi are off when not in use
-Disable auto backlight (Settings->Sound & Display) and set a constant, not too bright screen setting. This probably won't save you a whole lot of battery life, but the darker your screen can get while still being readable, the less battery it will require.
-Turn off Background Data (Settings->Data Synchronization->Google) DO NOT turn this off if you still want your phone to auto-sync with Google services (i.e: Gmail, Calendar and Contacts)
-Turn off wireless networks for location detection (Settings->Location) If you don't use location-aware apps, this should save you a bit of battery. Clear the top box; leave the middle one checked.
-Make sure that services that you use don't update too often. The common culprits are Twitter clients, the Mail app (not Gmail) and Weather. I tell Twidroid not to auto-update, Mail to update every hour and turned off Weather auto-updates completely. All of these can be updated manually, and having them fetching data constantly is a real drain.
-I've heard that the clock widget that the phone supplies by default (the HTC clock widget with the flip clock and weather under it) has some sort of memory leak. I switched to another clock that takes up less space on screen and doesn't include weather updates.
You probably have already tried all of these things; but I figured that I would throw them out there if you haven't already. I've been wrestling with my phone ever since I got it to squeeze the most out of my battery, and I think that the best I've been able to do is about 3% drain per hour with moderate use (Mail fetching every hour, manually checking Twitter every hour, occasional texting, and occasional Pandora).
Good luck!