Update: If you haven't seen the movie, beware of the comments. There are some small spoilers.
Wow! What a great movie! Jodie Foster is terrific and the plot is far more plausible than is typical for movies of this genre. Incredibly suspenseful. It's the best airplane movie since Airplane! Go see it immediately.
Roger Ebert agrees with me here:
How can a little girl simply disappear from an airplane at 37,000 feet? By asking this question and not cheating on the answer, “Flightplan” delivers a frightening thriller with an airtight plot. It's like a classic Locked Room Murder, in which the killer could not possibly enter or leave, but the victim is nevertheless dead. Such mysteries always have solutions, and so does “Flightplan,” but not one you will easily anticipate. After the movie is over and you are on your way home, some questions may occur to you, but the film proceeds with implacable logic after establishing that the little girl does not seem to be on board.
Stephanie Zacharek, of Salon, offers a different view:
The performance suits the movie too well: “Flightplan” is an exercise in edgy tedium, and even though it's only 90 minutes or so, it seems to last longer than an actual transatlantic flight. If you bring an eye mask and a few sleeping pills, you should get through it OK. A magazine or book wouldn't hurt, either. It'll be over before you know it.
Well, what can you expect from Zacharek? She liked the pathetic Fantastic Four but didn't like the magnificent Batman Begins, for heaven's sake.