I've noticed a similar drop-off, though not quite as dramatic. After I cycled the battery a couple times when new, it seemed to take a lot to put a dent in the battery life...now it doesn't take long at all to drop to 50%. I don't have anything running full time as far as 3rd party apps go, nor do I have the GPS active.
I actually wonder if it's an issue with the battery metering...it seems to drop to 40-50% pretty rapidly, and then take much longer to go fully dead, even after the green LED starts flashing.
Either way, I've done no actual quantitative analysis on this...just my impressions as I've been using the device over the last month and a half...