Well, the PSP plays all PSOne classics released so far. All of those games were able to function with the original PS1 controller (No rumble, no analog). As far as I know, Ape Escape is the only game for the PS1 that made having a dual analog styled controller mandatory. It won't work on the PSP so I don't see it ever coming to the PSN (lame reason but I still think this is why).
Anyway, there IS a PSP version available on the PSN. I forgot what it was called, though. I debated buying it but the thought of Ape Escape with only one analog stick makes me cry. *cries*. So there ya go. Hope that helps ya.