The distance it will go *is* the power level. A signal from a light detector in bright light will travel further than a signal from a weak light. To detect signal strength, you can use a comparator or place your devices at a certain distance along the redstone wire.
No, the distance it will go is how far it will spread. The power level is whatever the current signal strength is in any one piece of redstone. Also the whole idea of different power levels as it applies to a signal in redstone is completely new. Before the closest you could come is whether you where powering a block strongly like with a repeater or weakly with just a normal signal. Redstone used to be either off or no and now it has different levels of on so it went from black and white to a a gradient though one is still exactly off.
I'll try to explain it again: The power level and how far the redstone will spread are the same thing. I just loaded up the snapshot and tossed a few potatoes onto a gold pressure plate. The more potatoes I put on the plate, the further the signal went along the redstone wire.
The only reason redstone used to be 'off' or 'on' is because all available devices only sent out full-strength signals, and all available mechanisms responded to the weakest signal.
Now that there are devices that can send out weaker signals depending on stuff they detect, signal length is no longer just a limit on how far away you can put a lever, but also a way to detect how powerful the signal was, by putting repeaters or gates at a certain distance along the wire.
Feel free to download the snapshot and see how it works for yourself. It's pretty neat.