I recently upgraded to CS5. Maybe that as a stupid thing to do before the first upgrade was released... As usual, there are a list of bugs. One of which, it turns out, does not allow me to work.
When exporting any FLA, including a brand new one with no code whatsoever, I get an "Error opening URL" error. It seems Flash is trying to find the SWF I am trying to export, and is getting lost.
After deleting my preferences, reinstalling, checking forums, and all the other regular things you do to fix a bug, I gave Adobe Support a call. After an hour of getting to the right person, trying to explain to him the problem, and setting up screen sharing, it turns out File Vault needs to be turned off in order to export a SWF. I tested this by creating a new user account, and everything worked fine. It also fixed the issue with Dreamweaver CS5 not opening at all.
I can't go without security, so looks like I will have to use CS4 until Adobe comes out with a fix.
Hard to believe Adobe wouldn't have tested this before releasing.
Adobe seems to have fixed this. See blog post for more info