Against all the odds, Andrea Rossi's E-Cat cold fusion power plant passed its biggest test yesterday, producing an average of 470 kilowatts for more than five hours. (A technical glitch prevented it from achieving a megawatt as originally planned). The demonstration was monitored closely by engineers from Rossi's mysterious US customer, which was evidently satisfied and paid up.

The energy was output in the form of heat, measured by the quantity of water boiled off. The results are reported in NyTeknik and Pure Energy Systems News, who both had reporters present for the test. Associated Press also sent a correspondent who should be filing a story in the next few days (one suspects his editors might have some questions).

But this does not mean we can crack open the champagne and celebrate the end of fossil fuels quite yet. Skeptics have plenty of grounds to doubt whether the new test really takes us any further forwards.

For a start, the US customer remains anonymous. In other words, a group of unknown, unverifiable people carried out tests which cannot be checked.

Secondly, observers apart from the customer were only allowed to view the test for a few minutes at a time and during the entire test the E-Cat remained connected to a power supply by a cable. The external power was supposedly turned off; as a demonstration it would have been more impressive for the reactor in its shipping container to be visibly disconnected while operating.

The successful test should pave the way for further work at the University of Bologna, and more contracts with the enigmatic customer. NyTeknik did discover one possible clue to their identity. The customer's controller, one Domenico Fioravanti, apparently reports to a man whose title is "Colonel". This suggests that the mystery customer might be DARPA, the Pentagon's extreme science wing which, as Wired.co.uk has previously noted, has expressed interestin Rossi's work -- but which might not be quite ready to explain to its political masters why it spent millions on a cold fusion device.

Plenty of mysteries remain. But the game just got a lot more interesting.