According to iLounge, the best combination for looks, usability, and protection is using a NLU BodyGuardz film on the back of the iPhone with a Power Support Crystal Film on the front. This is the exact combination I use, and it is wonderful. Since then, FullBodyFilms came out with their Protection Pack, which included a BodyGuardz for the back and a Crystal Film-like front, for much less than buying the two separately. I sent FullBodyFilms an e-mail about the 3G, and they said they were "still testing" but that it would be out sometime in July. NLU has begun taking pre-orders for their BodyGuardz, which it says will ship July 12. ZAGG has begun shipping orders for their InvisibleShield already, but they've had so many pre-orders (at least 50,000) that if you order now it will be at least two weeks before it would ship. In my experience, the NLU BodyGuardz feel much nicer than the InvisibleShield, and they cover slightly more of the iPhone. Also, my experience with ZAGG was so terrible that they ended up refunding me for my iPhone shield purchase, and I've decided to never buy from them again.
I should note that when I took my iPhone into an AT&T store, the customer service representative really liked the look of my BodyGuardz-covered iPhone and asked me where I got it. Despite selling many iPhones and having his own iPhone, he had never seen anything like it. It gives the iPhone just a little bit of a glossy effect that makes it look nice, and it won't show fingerprints or smudges. Plus I really like that it doesn't cover up the sleek iPhone with cheap-looking plastic/rubber. After about 9 months, I began to notice the dirt that was accumulating along the corners began to make just a millimeter or two of the film begin to not stick as well as before, so I eventually replaced the BodyGuardz (it comes with two sets), but it lasted me at least 9 months, and with the included second set, I expect it would last around 18 months.
I'm pretty sure the size and shape of the front of the iPhone are exactly the same, so the Power Support Crystal Film (or any other static film) should still work for the 3G, but for complete back protection, it will be hard to get anything from NLU, ZAGG, or FullBodyFilms at the launch of the 3G. I like the look and feel (and protection) of the BodyGuardz/FullBodyFilms so much that I'm considering delaying purchasing the 3G iPhone for a week or two to make sure I have good protection first. I guess I do treat gadgets like children (I can afford that, since I don't have any real children, at least not yet), and I would hate for my brand new iPhone to get any scratches.