Joe,
You can use a 45 degree bit to cut a miter on a piece of plywood, but it doesn't always work that great. The veneer can occassionally fuzz or split, making a joint that doesn't look that great.
You can use a rabbet joint, which will hide all but a little end grain, which makes a fairly strong joint, or you can edge band the ply with hardwood, and miter the joint that way.