In component mode select vertexs along the seam you want to create.
Go to edit polygons>split vertex
Go back to object mode and select your object
Go to edit polygons>seperate
Deselect your object and notice that it is now two pieces!
Answer from Maya-Listserv on Highend3d.com (Michael McAnaney, Mon, 19 Feb 2001 15:25:23)