If Atlus really wanted to they could probably port the original Persona to the DS, but considering how badly it's aged in its original state, and that most interested have already bought the PSP Remake, it's not likely.
Persona 2: Eternal Punishment would pose more of a problem, as it was a pretty decent-looking PS1 game, and had a sizeable amount of voicework.
Considering they ported Disgaea I'd say anything's possible, but it's pretty unlikely Atlus would undertake such a project. On the offchance Persona 2 gets ported anywhere in the near future, it's either going on PSN, or to the PSP.
Not to mention if they ever wanted to bring Innocent Sin over, they'd have to translate the whole thing from scratch.