I have uploaded the source for Poly/ML 5.5.1 to the SourceForge site so it is now officially released. I still need to create installers for Windows. The release notes at http://www.polyml.org/docs/ReleaseNotes.html give a general overview of the changes over the last year. There have been no big changes which is the reason it's 5.5.1 rather than 5.6.
David