No, junior member, because I'm thinking with a bit of common sense. You don't take hold of any market in a second, like so many Sony fans are declaring in this thread. How is it possible for a new competitor to take complete control of a market and get the entire marketshare? It's not fucking possible, as much as you would like to believe.
Nintendo isn't doomed. PSP is a multimedia device that happens to play games. GBA is a pure handheld game system, which you cannot count out because NDS still plays GBA games and GBA sells more than anything other hardware at the moment. NDS is a multipurpose game and communication device. All three have different attributes to their advantages, and this is where the market will split. It won't split evenly, because, face it...PSP is going against Gameboy and Pokemon.
This announcement is exciting, but Sony has a lot to learn about the handheld market just yet.