According to Famitsu, Capcom chose to delay the game as a means to improve the quality of the product. A new release date has not been determined. Meanwhile, as of this writing, Capcom Japan nor Capcom US have yet to announce the delay on their respective Legends 3/DASH 3 communities. That news should be forthcoming, however.
Disappointing, to say the least, but hopefully this will lead to a far better game. We'll keep an ear out for the new release date, but I think it's safe to say Prototype will release after E3.