One of the features I am most excited about in Flash CS3 is the full support for exporting to a QuickTime movie. Timelines no longer need to be built specifically for QuickTime export, and you can animate through code, use ActionScript 'til your heart is content, and it all outputs cleanly to a QuickTime MOV file.
Well, in theory, at least. Over the past few days I have been trying to export a project to QuickTime, specifically the "Did You Know" video we are working on at XPLANE, and the progress bar always stalls at the very end. We tried letting it sit for an hour, a few hours, and even overnight -- all to no avail. We still haven't figured out what could be causing the issue, but I did manage to get it to work by setting the QuickTime export preferences to record for a length of time, rather than "When last frame is reached":
So if the progress bar freezes while exporting, try changing the setting to "After time elapsed". You can easily get the correct length in seconds by taking the total number of frames and dividing by the frame rate. It may be helpful to note that Flash CS3 itself never froze -- we could always hit the cancel button and it would stop the process immediately. In fact, every system we tested it on had the exact same issue and showed no signs of any processor or memory spiking problems.
Hope this helps others who may encounter the same issue!