And not just in flight!!! You really should keep the belt fastened until you're at the gate and that little warning light goes off, just as they tell you.

I know from personal experience.

Last October I was aboard an American Airlines MD-80 that hit a fuel truck just a few feet from the gate. (Search for the old thread "Ramp Collision at BWI" in Trip Reports if you want more details) My seatbelt was still fastened, and it was a good thing -- because the deceleration I felt would definitely have put an impression of my face into the bulkhead in front of me. The flight attendant was standing in the aisle passing out coats, and he did go rocketing forward. He was able to keep his feet, was uninjured and even joked with us afterward, (about seat belts, by the way) but I thought at the time that he had to be hurt.

So don't try to rush getting your backpack out of the overhead.