After updating to 4.1 we are not getting a true display of battery levels it seems... I see a LOT of threads on Apple's own forums about bad battery life with 4.1 (on all iPhones) but I wonder how many realize it's just a bad display of the battery life, or at least it is for me...
My battery can look almost empty but I don't get the 20% level warnings... Then later on it'll still show almost empty and just die on me. I can then charge it up again and it'll still showing almost empty even after charging all night.
Then I just decided to turn it off and back on (standard way, holding both buttons until I got slide to turn off) and somehow I went from an empty battery to a full one just by turning it off and on?
It may be a software or hardware issue. You can reset your iPhone's battery display by shutting the iPhone off and turning it back on. Another is to reset all settings (general/reset/all). Lastly is to restore from backup/new iPhone.
Note that battery life varies on how you utilize the iPhone. As many have stated on the Apple forums, they see the battery percentage drop rapidly from 100% to 0% on standby, but this can vary if you have loads of apps running on multitask or you have push features of multiple email accounts or apps active.
If your iPhone continues to have the battery percentage issue after resetting or restoring, visit your local genius bar for further help.