I have updated our ArcMap extension to work with ArcMap 10 - mainly just change reference from ESRI.ArcGIS.ADF to ESRI.ArcGIS.ADF.Local and set value of 'Specific Version' property for all references to false, and also use the new ESRIRegAsm.exe to register it. So I am wondering now if I can build a single extension that will work with both 9.3.1 and 10.0 or do we need separate binaries for each product?