I read somewhere that hidden power is just going to take the type and power of another move, and that the move that it copies is based on the species. It went on to say, as an example, that if a Zapdos were to use Hidden Power, it would behave as a water type move with a base power of 120, in other words, the move Hydro Pump.
Of course, I read this a long time ago, I think during the time of the Sliver/Gold/Crystal versions, and I don't remember where I read it either. So it might have changed since then, or just been wrong. But whatever:)