Visio can import and export AutoCAD 2000 (R15) format files. So if you have AutoCAD 08 make sure to save in AutoCAD (R15) format. This is a very good article on the issue of Visio and AutoCAD import export.
http://www.visguy.com/2006/10/26/visio-autocad/
When we say D-Tools is not compatible with AutoCAD 2008 it means that you cannot do SI drawings with the D-Tools UI like Project Data Matrix, Product Explorer etc within AutoCAD 2008 now (similar to our Visio UI). The D-Tools UI works only in AutoCAD 2006 and AutoCAD 2007 now. We are striving for AutoCAD 2008 compaitibility in our next release 5.1.