Oh, to throw my own two cents into the speculation for platform:
3DS: Very possible. The 3DS has been good to Atlus - SMT IV broke a quarter-million on it in Japan alone, both recent Etrian titles broke six figures, and for a remake, Soul Hackers did very well (didn't break 100k, but came within sneezing distance of doing so). Every single one of the titles they've released on the system has clearly made a profit and been well-received. And they have a 3D engine in place thanks to SMT IV - this could easily serve as a foundation for other titles in the way that the Nocturne engine provided the building blocks for a number of PS2 titles. And the 3DS is the fastest-selling game machine in Japan at the moment, and while it may not be on track to beat the DS, it will do at least as well as the GBA, if not better. While there may be an argument that the Persona titles have "always" lived on set-top consoles first and foremost, that could also be said for main-line SMT once upon a time, yet here we are. I would not be surprised at the announcement of P5 as a 3DS exclusive in the slightest.
PS3: The obvious choice. The Persona team's made a game on the hardware so the tools and pipeline infrastructure is there, they have experience with the platform, and they have a proven market of core gamers on both sides of the Pacific. The only thing that makes me hesitant to say this is a sure thing is that the PS3 is much more stagnant a console than the PS2 was, especially in Japan; with SMT IV going to the 3DS in particular, it no longer feels like a safe bet that P5 will be going to the PS3 as a matter of course.
PS4: Possible, but unlikely. The thing that makes this really unlikely is actually the fact that Atlus has traditionally waited some time before making generational transitions; remember that one of the reasons Persona 4 garnered so much attention was due to the fact that it was a PS2 game in the second half of the Year Of Our Lord Jesus H. Christ Two Thousand And Eight. Even first releases tend to be delayed; Nocturne didn't come out until the PS2 was three years old, the PS3 was five when Catherine hit, and even Etrian IV came out nearly two years after the release of the 3DS. I find it unlikely that Atlus would jump into the new generation so quickly, even with the financial assets of Sega now backing them up. It certainly remains a possibility, though.
360/Bone: To the idea of exclusivity I say merely: lawl. There is no way Atlus would create an exclusive for such a dead (or stillborn, in the Bone's case) platform, even if Microsoft threw mountains of money at them. They would not risk alienating their domestic playerbase to that degree. Catherine did come to the 360, so if the game goes to the PS3, it may see a 360 port for the overseas market; if it's PS4, a Bone port is a possibility due to the architectures being so similar. I find the idea of an exclusive impossible, however. Even if Microsoft offered to write a blank check with no questions asked for its development, it simply makes no economic sense for their domestic market.
TLDR: I expect an announcement primarily for the PS3, though I would not be at all surprised at a 3DS announcement. Anything else would raise some eyebrows.