IDEO (
www.ideo.com) and Frog Design (
www.frogdesign.com) are at the top of the food chain when it comes to this type of work.
I can point you in the direction of a very qualified firm that are a little more down to earth and affordable. They also have in-house design and prototyping capabilities.
If you would like their contact info, ICQ me at 168455758.