There is no backwards compatibility unfortunately... you could export an *.edl or *.xml and reimport it in PPCS5.5 - you will have the arranged clips then, but no effects if i'm correct...
Thanks Stefan; I was under the impression that although there's no official backwards compatibility, as long as u don't use any new features like warp stabilizer. Guess there's only one way to check, but if someone can verify ome way or the other that would be great!
I have both but cannot open CS6 bin CS5.5. I always get that saved in another version error message.
Have not tried the FCP XML route, but will not bother. In my experience so far CS5.5 is much more stable and faster than CS6, so....I'll probably uninstall my copy soon.