You should be able to get some real buys on "03 & '04 GT's now, because guys like me are trading them in.
I really liked my '04 GT, except for one problem. The ergos just didn't fit me well, I got severe neck pain when I rode it. That's probably my problem, not the GT, but it prompted me to consider the new GT. When I heard that it had an upright seating position, I took a chance and dropped down a deposit way back in December. Even after riding the pre-production '06 GT during bike week, I still had deep reservations about trading in my '04 GT. Except for the neck pain problem, I loved it. It was comfortable, plenty fast enough for me, and good looking.
I agree with all the things the others here have posted about how much better the new '06 GT is compared to my '04, but you could pick up an '03 or '04 GT for almost half the money of the new GT, and most of those used bikes still have factory warranty left. The old GT was nearly as fast as the Honda ST1300, and was a far better bike than the ST1300 in my opinion. So, it's not like you'd be taking a very big step back if you bought one. It's still a hundred forty MPH machine, and there's very few places that you can go faster than that on public roads!
If your budget permits, ride both and see if the new one is worth the money to you. If money is an issue, buy an '03 or '04 and enjoy the heck out of it! Either way, you can't go wrong.