Exchange 2010 calendar synchronization problem
Since I have currently the same problem, I found out that it is related to the import of a pst file.

The error occurs if a pst file is exported from an MS Exchange 2003 Server and afterwards imported on an Exchange 2010 Server. (e.g. in order to migrate from one version to another)
Everything works fine when using OWA or Outlook to access the data. The error you described shows up only on mobile devices so far. (In my case it is a Symbian phone with Dataviz roadsync on it)

Except editing all items, which makes them available again, I found no other solution so far. But I am curious to know if there is anything else.