The MIT Micro Gas Turbine Engine Project has developed a micro turbine rotor engine. See the Wired News article, The Little Engine That Could Be, by Louise Knapp. (Thanks John)

There's also a white paper written by the same group at MIT that explains how it works: Micro Electric Machines for Micro Turbomachinery.

Here's an excerpt from the Wired News article:

The development of a fuel-powered miniature engine, touted as a more efficient and longer lasting alternative for the battery, may push the Energizer Bunny to the unemployment lines.

No bigger than a regular shirt button, the micro gas turbine engine uses the same process for producing electricity as its big brother electricity stations -- burning fuel and running it through a power plant.

"Fuel and air in, and electricity out," said Luc Frechette, assistant professor of mechanical engineering at Columbia University and one of the team members building the engine.