It's really hard to write this post without editorializing, but I'm going to try to stick to the facts and avoid ranting about my feelings with regards to using Flash in open source projects ... really, I'm going to try...
If you are using, and rely upon, the "Flash Upload" functionality of WordPress or Joomla!, DO NOT UPGRADE TO FLASH 10, at least for now. Flash 10 will break that functionality.
On of the nicest things about the web is that linking enables you to avoid having to restate things others have already said, so if you want to get the fuller picture on this situation, here are some links that you will find interesting and that explain pretty well what is happening here:
Flash Player 10 Beta breaks file upload (26 posts) - from the WordPress Support Forums
Blog post from a Joomla! Core Developer
Now, how could this have been avoided? Well, if these open source projects had remained true to their open source roots and refrained from embracing proprietary ....
(oh yeah, I promised not to rant .... sorry!)
--DreamHost Tech Support