Not directly. If a project is saved from PP CS5.5 and subsequently opened in PP CS5, you will be presented with the conversion dialog box you see if PP encounters an older version of a project. If the project uses features which are new in CS5.5 (such as merged clips), they will of course not convert down to CS5 in their native form, so in the case of merged clips the will appear in CS5 as a normal A/V sequence both in the timeline and the bin. CS5's converter module makes an independent copy so it's not realistic to flip the same material back and forth between CS5 and CS5.5. If you re-open a CS5 project in CS5.5, you'll get the same conversion dialog box, this time asking to upsave a copy.
There's no direct "save as CS5" option in PP CS5.5, as the conversion tool at the remote end of the chain handles the process.
Not very friendly approach towards those who have not upgraded yet and you DO have project files to share with... IMHO, Adobe should approve on this. Not forever (e.g. CS5 to CS3) but certainly within a point-release (CS5.5 to CS5).
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