Someone down at the convention center where this year's E3 is being held was able to have a look at the show being set up. In the Rockstar booth, among other things, is a bus painted completely black with the text "Grand Theft Auto: Liberty City Stories" underneath the PSP logo. As no official name has been given for the GTA PSP title expected in summer 2005, this seems a strong contender.
This story seems even more convincing considering Rockstar Games registered the domain name LibertyCityStories.com on the second of May for the next 5 years.
Thanks to Joystiq and the webmaster of LibertyCityStories.net for the heads-up. Discussion is open at GTAForums' GTA PSP section.