They finally released screens and info:
It's actually a new game, not a port, set in the same city as GTA3 (Liberty City) although they had to rebuild it from the ground up to fit on the PSP properly, so the layout is a bit different. Even though it's gonna be running on a handheld, it's still gonna have fully streamed loading and a full set of radio stations.
There's a trailer on IGN but it's most just cutscenes instead of in-game stuff. You can find some in-game footage on the official site if you look hard enough, they're pretty hidden.
Released end of October this year irl.