I have recently installed MX SS4 (build 08.11.09.789) and when exporting to DWG all the lines are exporting as 3D polylines, even with the _2D MX files. This is causing a bunch of problems with our drawing production as the 3D polylines will not show linetypes correctly and all have to be converted.
I have checked the MX configuration so that the seed files are correct and the DWG export options do not have the linestyles representations ticked. I have also tried to have a look at the options available in the "save as" dialog box when exporting, all to no effect.