Very annoying if you ask me because I just want to work like I was used to without fla files growing in size with each new project (for no apparent reason).
Luckily I found a very simple solution: clear everything from your library you don’t need any more and then just hit “Save as…” and save as a CS4 file. Thiss step will remove most of the overhead. After this, saving it again as CS5 file will reduce the file-size even more!

This trick is a bit more work than before (and it would make more sense if Flash did it all by itself),  but atleast it gets the job done.

Hopfully an update for CS5 will get rid of this problem once and for all…

After reading Den Ivanov’s post I wanted to make a comment but it became so big I decided to make a post about it myself;

Apart from the bug Den describes there are tons more. I also have troubles with the panels who very often lose their “titlebar” in which case it’s impossible to close them or switch to other tabs in that panel. The only way to restore them is to close and open the panel (hit F4 twice).
There also is a bug in the sizing of the help window as it often resizes to the height of the screen making it impossible to read the complete content.

It’s a shame they release a version that is even more buggy than it’s alpha version. Flash 9 is almost the same as version 8 (apart from the as3 compiler) and yet they manage to screw up the IDE.
Too bad the alpha version of Flash 9 couldn’t handle components because it was much more stable than this buggy crap.

I really hope they fix these bugs (they are definitely not eastereggs like Den said) in the next update…