you can open version 8.0 files in 8.5 and 2008 infact forward compatabiliy has never been an issue, it's only when you want to go backwards it gets a little tricky.
Once you save a file in the newer version of Maya though it will no longer open in the older versions unless you either save it as a .ma file and edit the version number in a text editor like notepad or save it as an OBJ although OBJ is not always entirely successful.
Another thing to watch out for if you want to go back to the older version after is that if you use any new features that are in 2008 and not in version 8.0 it might cause problems with the file opening.