Skip and Giles are correct. Let's add a picture! (check out my 1337 Paint skillz)
This picture shows the path of the reaction which turns hydrogen and oxygen into water, and vice versa. First let's make water. The hydrogen and oxygen are sitting at a certain energy; let's call that 100. Both of these molecules are stable, and will not react under normal circumstances. In order for these two species to react, a certain energy must be put into the system. This is the activation energy, or A1 in the picture. Lets say that A1 is 35 energy units. Now the total system has 135 energy units (the peak of the "hill"), enough for H2 and O2 to combine to form 2 water molecules. These water molecules at the bottom of the "hill" have a combined energy of, let's say, 20. So, the system started with 100 units, and ended with 20, which means that 80 energy units must have been released in this reaction. In this reaction, something as small as a spark can provide the necessary activation energy, setting off a chain reaction (explosion) which creates water and a very loud bang. The bang is those 80 energy units being released all at once.
Now consider the backwards reaction: changing water back into hydrogen and oxygen. In this case, we go backwards along this curve. The water molecules have 20 energy units, and the products, H2 and O2, have 100. Thus, this reaction CONSUMES 80 energy units in total. Futhermore, the required energy to start this reaction (A2 in the picture) is 115 energy units. This means that the water-to-hydrogen/oxygen reaction is not energetically favorable, i.e. it consumes energy overall. Even if we consider that the hydrogen can then be burned, we still use more energy than we can get out of the system. This is why electrolysis is not a viable option for producing hydrogen.
METAPHOR TIME!!! If you don't like that explanation, consider this: a sledding hill.
Let's say you're a child, and you have this great hill which you can sled on. Would you rather start at point A, climb the hill, and then sled down to B, or would you rather start at point B, climb the hill, and then sled down to A? Probably not the latter. Why? It's more trouble than it's worth. Why hike up the giant hill, only to be able to sled down a little ways? That's not as fun as hiking up the short side of the hill, and risking your life by sledding down the long side
Well, nature hates hills even more than you do, so it sure as hell is not going hike up that big hill either. The payoff is just not great enough.
This is why none of the "miracle" energy sources are feasible: they pimp-slap the laws of thermodynamics. Anything that sounds too-good-to-be-true, is in fact too good to be true.
