5 - 30 minutos
iPhone 4 battery started expanding because of charging circuitry due to jailbreaking (at least according to the folks at the Apple Store).
Back panel was popping off, and pushing the home button resulted in color changes on the display.
Great, super easy - took less than 10 minutes.
Between iFixit and the Apple Store, you can get great troubleshooting advice, and the iFixit parts list is really cost-effective...