I do think you will be able to upgrade in '08, because those cars are going to be built (at least initially). Moreover, the same car is likely slated for '09 as well.
However, if the next build has a tough time selling, DC could pull the plug anytime during the '08-'09 build. In that case, you won't see Gen IV.
In fact, the more I think about it, the more I think we're beginning to see the end. The skipping of '07 is telling that DC is hedging their bets. That's because they're really not skipping '07, but merely delaying the build by a few months so that they can qualify the car as an '08 and then have nearly a two year period to sell the model. They'll do an intial build, feel for the market, and if sales aren't there, they have almost two full model years to clear out the product.
It is they type of move that a car company would make if they were worried about not being able to move the inventory. Forward date it as much as possible so that obsolecense is delayed as much as possible just in case you need to sit on it for awhile.
All the "special editions" to sell the current car are also signs that an end may be near.
Let's face it, DC probably loses money on the program and in this period of difficult times for auto manufacturers, why keep it? I think DC has already determined that the market has changed a bit and now demands a performance vehicle like the Firepower instead of the spartan Viper.
Hmmm.....