At CES 2008, there was a UMPC on display by Toshiba that purportedly ran on fuel cells.

An excerpt from Reghardware:

Closer inspection revealed the UMPC is not itself equipped with a fuel cell. Rather, Toshiba’s built a fuel cell unit into which the UMPC docks, just as it might sit in a desktop sync cradle. The fuel cell sleeve neatly matched the width of the handheld, adding just a few millimetres to the unit’s thickness and almost three centimetres to the height.

Toshiba has been dabbling with the concept of fuel cells for some time now, and several prototypes for mobile devices have also been discussed.

As display and processing technologies on mobile devices improve, the powering solutions have a big gap to fill to provide true mobility on these devices. Fuel cells are seeing accelerated development on that front.