The AD-OFMS by Alpha Data is one thin mouse. The name, which stands for "A Damn-Original Flat Mouse, Sucka," is a white two-button dealie that is a half-an-inch thick and weighs 2 ounces. There's a lot of pish-posh about resolution and all, but you've got to be thinking why you need a really THIN mouse and not just a small mouse. This mechanical mouse even has light balls, weighing in at considerably less than an ounce.
Peripherals: Really flat mouse [TechJapan]