Not sure why the original thread I commented on regarding this issue was closed...
I was having the same, widely reported problem of the 310XT data not synchronising. There was a file with an old date - 31 Dec 1989 - which seems to relate to a change of personal profile data for weight.
What I found is that the synchronisation worked just fine after I'd installed the express software on a Windows 7 virtual machine on my Mac. Fortunately the VM was able to use the ANT+ USB device so it connected with the watch and synchronised the data.
So, for me, this seems to be a problem with the Mac OSX version of the express software rather than anything to do with the watch or the garmin connect servers.
Has anyone else had the same experience of a sync not working on Mac but working on Windows?