"Atlus seems to like the Wii. Plus, by keeping the series on PS2 this long they've made it pretty clear that they're not fans of HD development costs..."
Atlus also has a deal with Microsoft to put their games on the 360, and I would say MS wants more than two very poor strategy rpgs out of it. Also, when they see these review scores, they will decide that like FFXIII and SO4 P5 is a game they should add to their jrpg collection and with a review score of 97 for the game -- whipping any jrpg in their lineup -- they'll be even more inclined to pay the price to get P5.
Also, Nintendo doesn't like to have such mature games on their systems and in a way the gameplay in the Persona games competes with that of their first party Pokemon titles, so they would be less inclined to want to release such a third party game on the Wii.
Finally, it could be that Atlus loves the Playstation brand explaining why they've had two top Persona games on the PS2 so late in its lifetime, and that they've just been waiting for the right time to announce P5 for the PS3. I would expect to see P5 on the PS3 before I would expect to see it on the Wii as well as Atlus would want to release the best possible version of the series on a next gen console instead of having a next gen game on a console that is decidedly last gen.
If saving money on development costs of the game is a big enough factor for them in the development of the next one as you say, then they could do pretty much the same thing by having P5 on the PS2 once again and save a few dollars in development costs from what they would have spent to make a Wii version.
Heavens to Murgatoids.
