ok thanks for the replies. it looks like either:
1. one of the batteries that shipped have gone bad or have less life
2. Mango needs calibrating
I've pulled it from the charger, and so far it is outlasting my work iPhone 4. My phone reads 100% (with 1 day and 18 hours left), while the iPhone is 99%. Both have WiFi on (with the auto sleep feature that activates to turn off WiFi when the devices are sleep that can't be turned off....grrr) and I've used my phone a tad bit more (for texting and such) and have my Gmail set to "As Items Arrive", whereas the work iPhone 4 only has Exchange on push (which my Arrive has Exchange on "As Items Arrive" also).
So....either it REALLY needs calibrating, or the other battery I had in yesterday was bad....or a mixture/combination of both.