I'm thinking either a double dado or single dado.

double dado
___________
|__
|xx|________
|xxxx|
|xxxx|
|xxxx|

single dado
___________
|____
|xxxx|______
|xxxx|
|xxxx|
|xxxx|

Got an interesting idea though. Use dowels at a 45 degree angle, coming all the way out both surfaces of the cabinet, out just far enough not to actually go into the cabinet. Then cut em off and sand flat.. Would definately help the joint, might be hard to drill at a 45, might look neato though.