I sorted it out.

It appears the Body property was never intended to accept RTF text, and Microsoft does not support it. This "feature" was introduced by accident, and there is no way of reading the RTF text directly from the mailitem's body property, or another property.
However, there is a dll available from Microsoft with which it is possible to read the RTF text of the body.
http://support.microsoft.com/?kbid=172038