The Gameboy brand will continue, but I doubt we'll be seeing another 'Gameboy' for quite some time. There were brief talks about a 'Gameboy Evolution' even after the DS was launched, but I really don't see Nintendo mentioning anything about it until at least 2 more years.
The DS is going incredibly strong. Part of its success is due to the 'Gameboy' brand, even though it's not labeled 'Gameboy'. 'Nintendo,' as a brand, is more recognizable than 'Gameboy'. The DS, however, was made to go in a different direction than the Gameboy. So that kind of makes me wonder... is 'Gameboy Evolution' gonna be more of the same? Hmmm....
I'll be quite fine with my DSes for a while.