The car is built on the Delta platform. Which Opel may use themselves, I don't know.
It shares it's architecture with the Saturn Ion (A car in need of a redesign in my opinion).
This car is supposed to be GM's premium small car at least in that size. It's designed to vie with Honda & Toyota. Considering that the current Cavalier is nowhere near Honda or Toyota or even Hyuandai in terms of refinement, powertrain finesse, styling, materials, etc.., I hope GM has learned some lessons and applied them to this car.
I will be in the market for a new car roughly about the time this car appears (Fall 2004 for the intro, SS probably next spring) so I'll pay close attention to it.
Considering that Ford is making a big mistake with their Focus (for the US market, Europe gets an all new design, while we get a "re-designed" Focus) which in most peoples opinion, on this site, is rather dull, the new Cobalt might just fare okay.
I can't wait to see one in person.