At the time, PlayStation.Blog Europe confirmed (in a forum reply to user Bahamut44-4) that the content had been removed at Activision's request, and that it "likely" would not return. Fast-forward to present day and things have changed, for a limited time.
The previously released (and subsequently removed) DLC has now been re-released on Xbox Live Marketplace and, according to Activision, the PlayStation 3 version of the DLC's re-release is "coming soon." Fans who missed it the first time will have until December 31, 2010, to download the content as it will be pulled in the new year.
The primary DLC pack includes playable characters Magneto, Black Panther, Carnage, Psylocke, and Cable, and costs 800 Microsoft Points ($9.99 via the PlayStation Store). Playable character Juggernaut also returns as a separate download for 160 MS Points ($1.99 via the PlayStation Store).