Found this in the Help. Seems you can't turn it off, but you can change the appreance. Thanks Autodesk. This ranks right up there with not being able to save the Expert System Variable anymore!!!
PREVIEWEFFECT System Variable
Concepts Procedures Commands
Type: Integer
Saved in: Registry
Initial value: 2
Specifies the visual effect used for previewing selection of objects.
0
Dashed lines (the default display for selected objects)
1
Thickened lines
2
Dashed and thickened lines