Tutorial/Tip: Saving From After Effects CS5.5 to Older Version/CS5
Not only can you open an After Effects CS5 project in CS5.5, a new feature to CS5.5 is the ability to save your project back to an older version (CS5) using "File > Save As > Save A Copy As CS5". However, new features to CS5.5 will not be retained in this transfer. In this Adobe TV Episode Chris Meyer discusses this new feature, and tips to avoid conflicts in your project files when doing so.
"After Effects CS5.5 includes the long-requested ability to save a project file back to the prior version. Chris Meyer demonstrates how this interacts with the new features added in CS5.5, including the ability to remember CS5.5-specific effects."