There have been a couple of threads on this already, but I'll repeat what I learned from ManaByte.
Open the movie in QT Pro and isolate the clip by dragging the triangles around on the slider, go to file>export>video as image sequence. It's a good idea to have a folder for gif so it keeps all your frames together. Open your favorite gif editor (I Use Macromedia's Fireworks) and import an image sequence as an animated gif. It's a good idea to set the timing to 8/100ths for better display in all browsers and cut out every other frame to compensate for the timing and keep the file size relatively low. Re-size, crop, and export as an animated gif and you're ready to rock.