Edit: Ravidrath got a detailed explanation. See post 86
Despite landing on the PlayStation 3 back in late November, Skullgirls' Slightly Different Edition update has yet to make its way to Microsoft's console. The delay is due to the file size of the update itself, according to XBLA Fan's translation of a tweet from the official Japanese Skullgirls account.
While Microsoft limits XBLA updates to just four megs, the fighter's Slightly Different Edition patch weighs in at 590 megabytes. Being granted an exception to the rule is apparently possible, though subsequently time consuming. No announcements have been made regarding an expected completion date for the whole deal.