Wait, what, Persona 4 Arena has a plot? Is "kicking the snot out of each other" not premise enough? Atlus spoils you guys.
Persona 4 Arena arrives in the US on August 7. A European release has been confirmed but not dated; it's expected to follow pretty shortly, but be patient, as both the PlayStation 3 and Xbox 360 versions are region-locked. Happily, multiplayer servers are global.