There has been much speculation regarding just which studio is developing GRAW2 for PC, as early press releases only noted Red Storm Entertainment and Ubisoft Paris.
Thanks to member Papa6, evidence has been found showing GRIN is officially returning to create the sequel. Details can be found in this forum post. It certainly seems official!
EDIT: HangtownSOG, the Ubisoft Community Manager for Tom Clancy games has just confirmed, via email, that GRIN is the defacto developer for GRAW2PC. Thanks!