A sun-bathing device that tells you when you're fried to a crisp? Please.
Sure, Minox's "Suntimer" has sensors that indicate UV intensity. So does something else--it's called skin. And yes, sunbathers can "program their skin type and the protection factor of sunscreen in use for accurate readings," as MobileWhack says. We have an alternative suggestion, shade.
Those who insist on getting one of these may need help figuring something else out: The strap could leave some unsightly tan lines. There's also this thing called melanoma, but that's probably information overload for these consumers.