Why is it "absolutely brilliant"?
It's not a bad idea, but so far I haven't seen any games previewed that seem to make particularly good use of the dual screens. It's just fluff at the moment. If the devs aren't gonna make serious use of the 2nd screen, I'll stick with my much more portable portable, the SP.
Sony's PSP, on the other hand, has enough of my interest that I'd be willing to take a step backwards to a larger form factor. If the battery life is tolerable, I'll be purchasing one of those.