This is kind of obvious but still worth to keep in mind. AT&T will let you do with your old iPhone after upgrading to your new iPhone 3G what ever you want.
You can hand it down to a family member or just have it deactivated and use it basically as an iPod touch.
After deactivation the Wi-fi and media player features still work.
It is important that you upgrade the original iPhone to firmware 2.0 before deactivating it. Upgrading might not work afterwards.
More details on Apple Insider.
See also this report about the AT&T launch and pricing details for the iPhone 3G.
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