It appears another invasion may be imminent, Commander. A rather large set of new achievements for the PC version of the game seem to suggest that more content is on the way for XCOM: Enemy Unknown.
2K recently confirmed something XCOM-related is on the way, and a ratings board leak implied it will be called "XCOM: Enemy Within." We haven't been sure if that would be a true sequel or merely an expansion, but these achievements give more weight to the latter possibility.
Shacknews has contacted 2K and will update as more information becomes available.