Because this API is internal to Media Center rather than publicly documented (by Microsoft anyway!) it means that they are free to change it whenever they like without worrying about backward compatibility. There are likely to be differences between the API in MCE2005 and Vista Media Center (and also between the schemas of the databases on each platform).
If you are planning to use this API in a commercial application, be aware that any Windows Update could potentially break your app, and leave you needing a version for the "old" API and a version for the "new" API.
Cheers,
Andrew