I use Juice Defender Ultimate on the Extreme setting and get about 2.5 days out of a charge unless I'm really using it heavy, but even then, never less than 8 hours. Your case seems pretty bad though. The stock battery is a 1500 mAh lithium-ion pack, and while that's not a huge capacity, it ain't too shabby either. Li-Ion batteries typically don't require conditioning, nor do they develop memory. That leaves just a couple of culprits: A process that is running and using a lot of power, like a GPS, or the phone searching for a signal when it's in a poor coverage situation, the phone itself has some physical condition causing high current drain or the battery might be bad.
You can check the processes by going into Settings -> About Phone -> Battery Use.
Checking current drain is kinda tricky, coz 2 hands just ain't enough to hold everthing in place. Plus, now that I think about it, I don't know where one would get the current specs for the phone! Let's cross that bridge if and when we need to. We might have to poll a few folks to see what is typical, and use that as a baseline.
That leaves the battery itself. Do you remember ever dropping or jarring it? If so, a solder joint might have been cracked and you might only be running on some but not all of the cells inside. You'd have the right voltage, but not enough current capacity.
HTH,
Paul
I'm curious to what it is myself.