Well, I tried it and it nearly bricked my phone for some reason. First let me say, that this is a brand new, 1 week old iPhone 3G--and maybe that had something to do with all the problems I had jailbreaking.
Man, I don't know if ATT or Apple-- or both changed something on these latest phones or what, but I couldn't get it to work at all. QuickPwn install was sucessfull then the phone got stuck in an endless reboot at the Apple logo. It would sit at the logo screen for about 15 minutes then buzz and reboot back to the Apple logo, this contiuned for over 2 hrs. I had to "revirginized" it to get it to work again, but not with out major work.
Tried it first on my Macbook with OSX 10.5.5, then (after restoring with the original firmware) on a friend's Vista notebook. It was a total pain getting the phone back into restore mode and reinstalling the original 2.1 software and firmware with iTunes--thought it may have been bricked for good for a while. The 2nd time iTunes bricked the phone saying the Activation Info was incorrect. A couple of hours later it suddenly activated on its own (not connected to iTunes)
I'm pretty sure I didn't screw something up, since I jailbroke and unlocked my old Gen 1, 4GB iphone to give it added eBay value. I followed the exact same guide I tried on the 3G (except for downloading the correct, First Gen. v2.1 update) with no problems at all. I was so impressed with the added functionality jailbreaking gave that I wanted to give it a try on my new iPhone. I still can't believe all the trouble I had!
Yikes! I think I'll leave it alone for now.