The issue appears to be that Access 2010 does not have its own file format. When you add a feature that is specific to A2010 (like the navigation control, data macro, or calculated column), that file becomes forever unreadable to A2007, even if the new feature(s) are removed. The solution appears to be:
- Remove all the new features from the file in A2010
- Create a new, blank accdb format database in A2010
- Import all the objects from the old database to the new.
I'm told that it is also possible to make the original file A2007 readable by removing the A2010 features and then making a minor edit to the file in a Hex editor. But I'd be wary of that approach. Importing the objects to a new database is safer, more reliable, and frankly simpler.