The Xpal AP1200 iPhone 3G and 3Gs Protective Case with Rechargeable Battery will protect your iPhone and also keep it charged as well. It can increase your talk time by 20 extra hours and standby time by extra 255 hours. You can have it at Woot, $39.99.
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Bought one of these XPAL AP1200 on Amazon for under $14 delivered and they are great. Now my battery on my iPhone 3G last twice as long as before.