I swear I keep having this dream where I'm playing my PSP and its controls have somehow morphed into a full-blown PS2 controller. This mod isn't exactly that, but it comes close.
This hack didn't require any hardware alteration in either the PSP or the PS2 controller - just a custom-made plug-in that enables the controller usage. Perhaps more impressive is the guy playing Full Auto 2 with one hand while he films with the other.
PS2 Controller Hacked To Control Your PSP [ubergizmo]