It looks like Apple shipped the iPhone 4 with the battery percentage display disabled. That was one of the things (aside from the gorgeous screen) that we noticed when we synced our iPhone 4. The instructions below were initially posted for the iPhone 3GS using iOS 3.1, but we felt that the new high-res screenshots coming out of the iPhone 4 warranted an entire how-to just for this platform.
If you have any comments or questions, please post them below.
Note: I used an iPhone 4 running iOS4 for this tutorial.
1. Tap on the Settings icon in the Home Screen.
2. Tap on General in the Settings page.
3. Tap on Usage in the General page.
4. Toggle ON Battery Percentage.