The name of the movie is "Chaos Theory" and, according to IMDb, it's now in pre-production (supposed to start filming in early 2006). Sounds like Ryan Reynolds is the star of this one and Stuart has a supporting role (no character name was given at IMDb; their info on the film is rather sparse). There's a brief film synopsis on that page (linked above). Also, since we're talking about Stuart, here's IMDb's profile of him.
There's an interesting mini-biography of Stu linked on that page. I wasn't aware that he was Irish (does he speak w/ an accent, IRL?) Just recently turned 33 (on Dec 15th). I also wasn't aware that he and Charlize Theron were an item.
I saw the word "fiancee" used over there(IMDb) so I guess that's means they're...engaged? I suppose it does.