Seems to be official.
How was the DS version?
I know it sounds like an overstatement, but it really was incredible --but it was incredible as a DS game. Keep in mind that they rebuilt the game from the ground up to take advantage of the DS (which could never handle the graphics in GTA:LCS or VCS --I enjoyed those when they were ported to bargain PS2 titles). GTA:CTW took the time to come up with an interface and ideas that took advantage of the DS's feature, like the stylus and mic, by integrating them into completely logical moments (i.e. instead of a cell phone, you have a PDA), while also having exceptionally tight regular controls. The game wasn't nearly as epic as the console versions, but it kept all the best parts of GTA (fun chaos, witty writing) and it was amazing how much they fit on the little cart, so it can't be fairly compared to the console versions. Thus, going back to my original comment: it was incredible feat on the DS.
That's why it was the highest reviewed game ever on the system.
From what I've read, in the completely ludicrous world we live, GTA:CTA was deemed a disappointment for Rockstar (or at least its shareholders), so this is probably being done to try and increase owners for the real decision makers in these large companies --people who don't give a shit about the game but care about the number sold.
Here's a warning: If this is just a quick push of the DS version onto the PSP, I don't know if it will actually be very good...