The battery life on the iPhone 3G and 3GS wasn’t very good, I pretty much used both of them with one of those battery cases. The battery life on the iPhone 4 is very good so you may not need to use a battery case for it all the time.
If you are a heavy user of the iPhone 4 and want some extra battery power then you may want to check out the Exolife iPhone 4 battery case.
As you can see from the photos the Exogear Exolife iPhone 4 battery case looks pretty good and doesn’t add to much bulk to your shiny new iPhone 4.
It features a 1500 mAh lithium polymer battery, and according to the manufacturers it will ‘virtually double the battery life’ of the iPhone 4.
The Exolife iPhone 4 battery case is available to pre-order for $90 from Exogear.