I've had my Droid for nearly a year now and am on Sapphire 1.1.1. Sometimes the battery level displayed will drop 20-30%. What's to blame? Battery or Software?
Too many possibilities without knowing your usage patterns, kernel, and overclocking settings.
In addition seeing what takes up the most usage in the built-in Battery Usage screen, you can use JuicePlotter to get a better idea of exactly how long your battery lasted at each state. The Spare Parts app will also give you access to Battery History so you can see what percentage of the time your phone is "Running" (high means something is preventing it from sleeping) and other useful info. The Watchdog app can help you monitor if any app is taking up too much CPU at any time. Finally, you can check out this thread in the Rescue Squad section to see if maybe your phone doesn't like your OC settings:
http://www.droidforums.net/forum/rescue-squad-guides/47871-overclocking-101-a.html
Another possibility is that your phone needs to recalibrate to your battery. I think the following method will allow you to do that: With your phone fully charged and still plugged in, reboot into Clockwork (assuming this is what you use) and clear battery stats. Then boot back up and unplug and use your phone throughout the day, letting it run down low. Then charge up again and see how you well it reports usage.
If none of these point to an answer, then it could indeed just be your battery wearing down. Things like numerous charge cycles, heat, and leaving your battery at completely empty can all increase wear time.
