It might be interesting to know that Andrea Rossi has a history with "breakthrough" discoveries and inventions.
His former company Petroldragon was disbanded after they dumped over 70.000 tons of toxic waste (and probably their method of recycling oil didn't work at all).[1 -wikipedia][2- italian source].
Most of the tests that were done violated very basic scientific rules and can be considered demonstration more than anything. The paper on arxiv.org was done by a scientist who was very close to Rossi (Giuseppe Levi) and some of its methods are questionable according to [3 - german] and [4- shorter/incomplete English version].
A real independent proof of the claims made by Rossi is still to be delivered.
While I hope that Andrea Rossi has found the next BIG thing and that he is just paranoid and incredibly bad at communicating, I seriously doubt it and go with "It's a big scam" for now. With some luck history will prove me wrong.