If it has to be Flash, then Sorrenson Squeeze seems to be best option.
There are two versions of the software package. One version is just for Flash, and a more
expensive version(Compression Suite) will do Flash, Windows Media and many of the same formats that Compressor will do.
Frankly, when compressing for broadband, I've gotten better quality with QuickTime/H.264. I've gotten a lot af great feedback from clients about the quality of the QuickTime video. As a result, I use QuickTime/H.264 whenever possible, and I use Flash only when compatibilty is an issue.
One note, however, the QuickTime H.264 requires QuickTime version 7.