"Hangover" star Zach Galifianakis was confirmed as one of "Due Date"'s two stars alongside the earlier announcement. Now we know who will join G on his shenanigans-filled road trip. Variety reports that "Iron Man" star Robert Downey Jr. is on board.

If you're surprised at the sudden addition of an a superstar like Downey, remember that Phillips scored a grand slam with "Hangover." The May release only recently breached $240 million in box office receipts, surpassing the Eddie Murphy classic "Beverly Hills Cop" as the highest-grossing R-rated comedy of all-time.

In "Due Date" Downey will play an expecting father who is rushing home to witness the birth of his first child. Galifianakis joins him in his journey at some point, and plans go awry. It sounds a little bit like the classic Steve Martin/John Candy comedy "Planes, Trains and Automobiles," certainly not a bad source for Phillips to draw inspiration from. Downey's dad-to-be will presumably be the straight man to Galifianakis's goofball.

"Due Date" is set to start shooting on September 21, presumably for a summer or fall 2010 release.

Are you sad to see "The Hangover 2" pushed off into more nebulous territory now that "Due Date" is in the works? What do you think Downey's presence might do for the movie? For Phillips? Might we expect an Iron Man cameo in the "Hangover" sequel now?