1. iTunes Gift Card - The one thing that iPhone users consistently spend money on since the App Store opened in July is applications. Although many of them are free, most of them cost anywhere from $.99 to $9.99 and beyond.
So, with an iTunes gift card they can get that $9.99 SimCity game they wanted or the $15.99 Beejive instant messager or the $19.99 Beatmaker drum machine. Or, they can buy songs, television shows or movies on iTunes. The iTunes gift card is VERY flexible. I have seen the iTunes gift card at Best Buy and Wal-Mart, although you can probably get one at the Apple Store.
2. Apple Store Gift Card - Many iPhone users like to get accessories. With an Apple Store gift card, they can get their own case, dock, or headphones. I would not get iPhone accessories for someone unless I knew exactly which one(s) they wanted. An Apple Gift Card will make sure they will get what they want.
3. AT&T Gift Card - AT&T bills are not cheap, so any help someone can get to pay their bill would come in handy. Also, there's a slew of iPhone accessories at the AT&T store as well.
If you want to GIVE someone an iPhone 3G for Christmas, the BEST way to do that is to get the iPhone 3G Gift Card from the Apple Store. It's $199 for the 8GB iPhone 3G and $299 for the 16GB iPhone 3G. Since activation is most easily done in-store, the gift card makes it much easier to give as a gift.