They're sticking their big guns on the 360, PS3 and Wii. It would be suicide to launch a console where a lot of their franchises could be found on competing systems.
And to be honest, with all the hype surrounding the current 3 next gen consoles, Sega would be butchered if they were to release their follow up to the dreamcast out of the blue.
It's wishful thinking, and nothing more I say.