@ shaun (third party battery replacement available)
Apple is not the only source to change the iPhone battery (incl. 3G). You'll need to make up a new excuse.
From macnn.com:
Milliamp has announced a battery replacement service for the new iPhone 3G. "Although the new iPhone won't hit the U.S. market until later this week, we wanted the world to know that we have the capability to replace the battery in the new iPhone 3G," said Anthony Magnabosco, the owner of the iPhone battery replacement service provider. Milliamp LTD has been offering battery replacement kits for practically every iPod model, as well as battery replacement services for the more difficult-to-open iPods for three years.
http://www.ipodjuice.com/iphone-battery-replacement-products.htm