Saw "Flight" starring Denzil Washington last night.
Would it be possible for a commercial aircraft to have been flown inverted or was that pure fiction?
Generally, no. I haven't seen that movie so I don't know what maneuver was proposed, but aerobatic (or acrobatically competent) aircraft tend to have high thrust/weight ratios, have high structural integrity (can take high force loads), and frequently straight wing design (i.e., more like the Spitfire* and less like the Concorde). But if you can keep g-loads down, and you have enough altitude to work with, even a heavy airliner could potentially do some upside down flight**. (And by "do some" I mean recover without structural failure and before ground-induced sudden deceleration)
But you can definitely fly upside down with very low g-loads, if you know what you're doing
It also helps that Bob Hoover flew like a god among men
*The Spitfire may be the most beautiful wing ever designed for its purpose, but was very expensive to design (all elliptical and near-elliptical designs are). For reference, the F22 wing is almost a modern version of the elliptical design, although some of this design is for low radar signature as well as low drag
**although I've been told that many commercial airliners could never do a barrel roll and at best could accomplish a split S if they were ever upset and inverted