A friend sent this to me to verify its authenticity, and after reviewing several websites it seems to be legitimate. Basically the radiowaves cause the molecular bonds to become unstable, which causes the hydrogen molecules to become volatile and combustible. The hydrogen can burn at temperatures up to 3000 degrees F (which melted several glass test tubes). In the current experimental setup, more input energy is required than can be extracted from the heat produced, but that's not to say it couldn't be made more efficient, especially on a larger scale.
You can read the HowStuffWorks article here.