I suspect resale is not going to be great, but trade value could be abysmal, especially at the Pontiac dealer.
Right now many Pontiac dealers (at least around here, and I would assume many places) are still overstocked on 2004 GTO's which they are trying to give away from $25K-$27K. Say you buy one. Next year you come back and want to trade it in on a 2005. Working against you first off is that these cars are not big sellers new with huge discounts, so a used one will be even more difficult for the dealer to move without a significant price drop. The second thing that will hurt is the 2005 itself. The LS2 motor in the 2005's alone will also hurt the 2004 MY's resale value. If I were buying next year, it would take a big financial benefit to take a used 2004 LS1 over a new 2005 LS2. The Pontiac dealer will not give much up in trade for a low demand car that is already antiquated by a revised model. You would lose a ton of money!
The 2005'swill be out in just a couple months at most. If you really want an '05, just be patient. They are on the way!