US retailer listed MAG 2 for 2011
A well know US retailer Walmart has listed a sequel to MAG, called as MAG 2. The store has given out the release date to it as January 2011.
Zipper Interactive the developer of MAG hasn't announce any sequel to the game in the past and they are currently completing their work on SOCOM 4. There isn't any official confirmation regarding the listing of the sequel, so take the news with a pinch of salt for diegestion.
MAG was released in January 2010.