Definitely Monster Hunter Portable 2G is the proof that the right games on the right time can sell very well on PSP. The hype behind Monster Hunter Portable build itself slow, but surely. Then we saw the first boom with MHP2, and this G interation is only the confirmation that PSP has found its Must-Have franchise in Japan. The bad thing is this game will have no effect in occident, where the DS is still selling like crazy. Things seems to have changes only in Japan with the Slim version.
At this point, I suspect taht Nintendo, even after 22 millions DS sold, should start to think that it's time for a redesign or a big games like NSMB2.
Nintendo dropped the DS support too quickly in favor to the Wii and I suspect that PSP is somehow benefiting from that. The fact that DS software that i selling is for a big part formed by games that came out 1-2 or even 3 years support this theory. They are like Super Mario Bros for the NES, but even the NES needed Super Mario Bros. 3 to win the battle against the new console from Sega. In other words, the DS needs a big Nintendo game. One is enough in my opinion is this one is a 2D Super Mario Bros. If Nintendo choice to wait and see, Third Parties, after MH success, could start to develop massively on PSP. It could be, since PS3 software and hardware sales are so pathetics.
Of course Dragon Quest IX is the life insurance for DS for this year, but a strong PSP means also that Square-Enix could start to think that a PSP port would be soo profitable. And we all know that Square-Enix behave like the wind.
In other words, only because Nintendo is now the leader, it doesn't mean that Sony is out of the race: a come-back is still possible (PSP Slim proved it hardware-wise. Software-wise: not yet, even with MHP2G) and if they keep seeing themself as unreachable, things could change once again...