It bites, doesn't it? I'm on CS4, but so many things have changed between each version it seems. I think you can simply choose in your preferences to work with Actionscript 2 and an older version of Flash. At least you can do that with CS4. Surely you can with 5, too. I'm sorry if this doesn't help, but that'd be my first thing to try and do.
Thanks for the ideas!
When I try to open a CS4 fla containing a slideshow with CS5 it gives an error message "Slides and Forms documents are not supported in this version of Flash. Please open in a previous version."
I called Adobe support and was told I would have to reauthor the slide show as a frame based app. After working for several years creating slide based flash presentations this is not good news.
I guess my fear now is that if they dropped support for slides and forms in CS5 will they drop support for them in newer versions of Flash player as well. Yikes! That would sink me for sure.