Square Enix has confirmed that the next Final Fantasy to see a re-release will be Final Fantasy V. It's planned for release on PSN as a PSone Classic, which means that you will be able to play it both on your Playstation 3 and a PSP.
"The PSOne version of the game has full FMV cutscenes and features old-school Final Fantasy combat using the Active Time Battle system," Square's Jem Alexander writes in the press release.
And that's more or less all the information available right now.