Akitio just announced their SK-3501 Super-S3 desktop hard drive enclosure, which wraps a 3.5-inch drive in an aluminum shell.
The stackable enclosure comes with a combination interface for eSATA, Firewire 800 (x2) and USB 3.0. Akitio uses a heat sink for passive cooling and integrated a perforated front panel to dissipate hot air. The design and support for USB 3.0 does not come cheap: Akitio charges $150 for the pretty aluminum case that may even look nice sitting next to an iMac.
For those who couldn't care less about the industrial design of the enclosure, but simply want the USB 3.0, there are options from about $30 on the street.