After having a bit of a dig into it, it turns out that Gran Turismo on PSP also has button cheat codes, like GT4 does.
Both are entered on the main menu (the menu with Single Player, Ad Hoc, Car Dealerships, etc.), and if entered correctly there will be a chime and the menu will reload.
All cars unlocked
Hold Start + Select + R and enter:
Up
Right
Left
Left
Square
Up
Right
Right
Up
Triangle
Right
Up
99,000,000Cr
Hold Start + Select + L and enter:
Down
Square
Down
Left
Right
Triangle
Right
Square
Up
Down
Up
Down
I've only tested them on the European release so far, but they most likely will work on all versions.
Both are entered on the main menu (the menu with Single Player, Ad Hoc, Car Dealerships, etc.), and if entered correctly there will be a chime and the menu will reload.
All cars unlocked
Hold Start + Select + R and enter:
Up
Right
Left
Left
Square
Up
Right
Right
Up
Triangle
Right
Up
99,000,000Cr
Hold Start + Select + L and enter:
Down
Square
Down
Left
Right
Triangle
Right
Square
Up
Down
Up
Down
I've only tested them on the European release so far, but they most likely will work on all versions.