After spending over a month on this, let me save you some time:
Exporting flash for youtube or such is easy. Adobe CS flash versions have movie export built in them. The trouble begins when you want to export movies that have more than a simple vector timeline animation, such as AS3.0 transitions, flashEff, and complex nested animations.
Basically there are three options:
1. Export a swf and convert it to a video format using a direct conversion
From my own experience NON of the many flash-to-swf converters i have tried (..and i tried many) could do the job. Always some bug get in the way..
2. Export a swf and convert it to a video format using a video editing software
Premier does not translate complex embedded swf files, some say After Effects will, but i didn’t try this one.
3. Export directly from flash to a video file
Flash’s native AVI export does include complex transitions (Flash CS4)
After spending weeks on this i can tell you that the only direct flash video export is using Quicktime Pro (in case of complex nested animation and flashEff; license is about 30$)
The QT export settings that worked for me (CS4):
File>Export>Export Movie>QuickTime (.mov)
check ‘store temp data on disk’
frame rate: original
Compression: Mpeg low complexity AAC
44.1khz, 16 bit stereo
If the video is intended for online streaming on youtube/vimeo, i recommend uploading the original outcome of this export and letting the hosting service to handle the compression, this way you will also have an HD version of your animation (if it was created as such).
Decent flash video export takes lots of RAM resources. In some cases you may need to break the export operation to a few sections and stitch them together later on. In this case one option is to use youtube’s online editor.