Yes, you can load the 2004 demographic data (USADemo.mdb) into MapPoint2006. With the caveat that some pinpoint and boundary areas have been deleted/changed/added.

You can't use both versions of the data at the same time without a little help. I have created some support files that allow you to do this, if needed.

It would take me about a half-day to create a new USADemo.mdb with both 2004 and 2006 data, if you're interested. I'd need to verify your ownership of both versions before supplying an updated file to you however.

-Paul