Rockstar president Sam Houser was quoted saying "Sony's extremely impressive technology has allowed Rockstar Leeds to do more on a handheld machine than we could have ever imagined", virtually guaranteeing that the game will be 3D rather than top-down. For what it's worth, we're prediciting a prequel to San Andreas based on CJ's life in Liberty City, probably featuring the likes of Donald Love and other popular GTA3 characters.
The reason for GTA's lack of loading screens is that it constantly streams off the disc - something that could be a problem, since a constantly whirring disc would drain the PSP's battery. We're expecting to see Vice City style postcards breaking the game into chunks.
One of the biggest draws to any GTA game is gunning down the enemy to a medley of tuneage - but again playing it direct from the CD could seriously sap the PSP's battery life. One suggestion is that the music could come on a memory stick as part of the package.
The PSP's a few buttons lighter than the DualShock 2, so Rockstar will have to come up with a way of corporating all GTA's controls. With the lack of a right analogue, we're expecting several fixed viewpoint options and a much more forgiving lock-on system.