TruePower IV Pro 3100 mAh Mobile Charger for iPhone/iPod
If there are any left by the time I finish typing this, note that it works reasonably well with the new iPhone 4. The cavity is sculpted with round edges to suit a 3G(S) but remarkably the 4 fits exactly with no strain on the connector and no rattle. It's just that it's only touching at the corners, so it would probably come out a bit more readily. If you can wait, there's probably something better coming, but if you need something in a hurry, by all means consider it.