I heard that Roush was going to stop selling modified Mustangs because it has become very hard to make money selling complete tuner cars. The 03/04 SVT Cobra really shot a large hole in the tuner business.
The way I heard it Roush is going to sell parts for the 05 Mustang, air dams, spoilers, scoops, suspension, engine, etc. but not the complete modified cars.