Sony only owns a small piece of SE, so it's not like they can dictate anything to SE about what systems they support, unless there were some provisions in the publishing contract for FFVII that keeps it Sony exclusive; but then there was also a PC version published by Eidos, so who knows.
If they rerelease FFVII, here's what I think is the order of likehood:
1) Direct release of the original game on PSN. Easiest way to get the game out and doesn't require an overhaul of the graphics that people would expect from a remake. I'm surprised they haven't already done this.
2) PSP remake: With a few FFVII tie-ins already on the system, they have the basis for a PSP version, and would require the least work of any remake.
3) DS port: SE could port it to the DS, which is comparable to the N64 in processing power, so no one would expect a total remake. The only issue is cramming all the FMVs onto a card; it might have to wait for a 4Gb card to be feasible, but they still have DS remakes of FFV and FFVI to do first (cross your fingers).
4) Wii remake (No "Wiimake" jokes, please): A full remake on the Wii would be easier than a remake for either of the HD consoles, also, the Wii is gathering a new audience that would never have played the original. One problem with making the game on a more "core" console is that most of the interested gamers would have played it already, and most would probably have had their fill of it; this could be why the PSP has only received tie-ins, instead of a remake of FFVII.
5) HD remake: It would probably be a PS360 multiplatform release, although sales of FFXIII would tell square which of these two systems is better to receive it.