If they are going to put out an H4 in '08, they better hurry up. It is off the books, and has been off all tracking reports for quite a few months. Look how long it took the H3T to come to life, and it is a minor redesign compared to a new vehicle with a new body and new drivetrain components.
If GM brought it back on Monday, earliest it could hit would be '09 and most likely '10. Have to remember, it has to be designed, modeled, approved, components chosen, tested (for off-readability, crash worthiness, emissions, fuel mileage, etc.), then put into production. If someone can do all that by '08, come on over to GM and get to work.
One vehicle I know of that was on the reports since '05 is not hitting the road until '08 and all it has is a basic engine change with a stronger rear axle, trans, etc., and some extra carbon fiber parts. The design has been around.