Reader Alex points us to this video where you can get that fancy iPhone 3GS battery indicator on any iPhone. Spoiler: your phone needs to be jailbroken.
We haven't tested it ourselves, but it looks like he's just using SSH to transfer some files over and create a new item to enable the battery gauge. Why this wasn't already included in all 3.0 firmware for all iPhones is beyond me. [Thanks Alex!]