The Invisual iPhone case by Critica is unique, having been designed for the blind and visually impaired. It encloses the entire device and features bas-relief buttons that coincide with a modified home page (you'll have to shuffle some icons around first). The coolest thing is that Critica claims it doesn't interfere with the screen's touch sensitivity or performance at all. The high contrast when the backlight is engaged will benefit the visually impaired.
It's just a prototype right now, so no release or pricing information is available. Critica has also stated that a forthcoming application will contribute to its functionality. If we hear any more about this unique case, we'll let you know.
Back in April, AT&T announced the Text Accessibility Plan's (TAP) for iPhone and iPhone 3G, which was designed for deaf users as well as those who are hard of hearing.
[Via Apartment Therapy]