A while back, you may recall this pack was set to release on December 20th. However, the pack was delayed and Magneto was removed. While there was no formal explanation for this choice, many suspected it had something to do with his likeness to King Juan Carlos of Spain. In any case, the DLC was later redated for a March 6th release.
In addition to the releasing next week, the pack will also sell for $1.99 or 160 Microsoft points. This is quite interesting as the pack was originally set to sell for twice that.