Atlus' oddball announcement of the evening had to be Persona Q: Shadow of the Labyrinth, what looks to be a 3DS dungeon crawler mash-up of Persona 3 and Persona 4 characters. While the exact gameplay of Persona Q wasn't clearly shown, it did appear to feature some sort of random battles or turn-based combat system. Also of note is the stylized look of the game, giving a chibi look to each character.
Overall I'd say the game sounds quite a bit like Capcom's Project X Zone, both of which introduce a couple of new characters whose adventure leads them leads them into meeting characters from other games run by the publisher. Of course, Atlus' may be quite different than that, especially considering the Persona titles fit into the same canon. Will Persona Q turn into a serious release or be a silly side-project focused on fan-service.
Persona Q: Shadow of the Labyrinth is scheduled for release on the Nintendo 3DS starting June 5, in Japan. Expect a western localization sometime in the month afterwards.
Thanks to @Rabla for the heads up!