This report from the BBFC, British Board of Film Classification (I think its a British system that rates the content in trailers/intros/outros etc) is showing a release date of June 24, 2011. It makes no mention if it is a DLC or separate disc sale. It does note that there is additional ~6 mins 29 seconds of additional cutscenes. Usually I wouldn’t post this kind of speculation, however it does seem pretty legitimate at this point and the site also mentions that more info would be released on the site 10 days before release.
BTW, it was also revealed in the latest arcadia magazine that Oni Akuma will be unlocked in Arcades on April 8th! Click through for the proof: