Stuart Beattie sure is one busy guy. Not only is he in the works on the Gears of War movie and in talks with Spielberg for the Halo movie, he is also revealed to have penned the script for the Splinter Cell (Conviction on Xbox 360, PC) movie. Now that's a gaming fan.
Okay, but all the aforementioned films have yet to be filmed for real - they're either on hold or on the renegotiation table. Still, you need only look at Beattie's portfolio to have an idea what kind of work he does. He was part of G.I. Joe: The Rise of Cobra, Pirates of the Caribbean, Australia and Collateral.
If you're still not convinced he can pull off a Splinter Cell screenplay, then just judge it for yourself. Script can be found here.