I'm not saying it's impossible that GTA PSP will cause a sales explosion, but betting that GTA PSP will do more for the PSP than Pokemon D/P does for DS is just fucking stupid.
Yes, GTA will have big sales in the US and cause a PSP explosion here most likely, but Pokemon is still big and will do that worldwide.
I think some people here forgot about what games are coming out for the PSP with-in September and after.
Here's the ones I know of
GTA, Madden - Will sell huge in america, but are not the major movers in Japan that they are here.
Tengai Makyou 4 - The best game on either of the handhelds, hands down. That said, the TM series is waining in popularity in Japan majorly (TM2 and 3 didn't sell too well.)
Shin Megami Tensei: Devil Summoner - Another awesome game for PSP, if a flawed one (and if they fix those flaws in the remake it'll be beyond awesome.) I don't know how well the SMT series sells in Japan though or if it's really a system mover, but I'd guess you'll see somewhat of a sales spike for it.
Final Fantasy 7 Offshoot (Not the movie) - If the rest of the FF7 games do well, so will this one. We don't really have too much data yet on that, but it DOES have the FF name and will probably sell a million (Did any PSP game in Japan sell a million yet, I don't know.) Probably another sales spike.
FF7 AC - I don't know how well UMD movies are doing in Japan, but given the choice between a DVD and UMD movie, I'd have to choose the DVD that I can watch on my regular TV.
Ys 6, Breath of Fire 3 - Again, I'm not sure of the popularity of these in Japan, I'll probably get them, but everyone is not me, we'll know soon enough I guess.
Bleach 2 - Well, the first one and the GBA one sold, but I don't know if we'll see a system sales spike for this because most of the people that would buy a system for it already have it for the first one.