I've never really noticed this on other vehicles, so I'm not entirely sure if this is normal or not, BUT:
On our '92 Tempo w. 175,000 km on it, I was changing the spark plugs and then fired up the car to make sure it ran ok. Then, before closing the hood I wanted to double-check if the cables were seated properly. So while the engine was still running, I barely touched the cable for the one plug, and was eletrco-shocked. Didn't feel like trying the others...
I don't normally do anything that has to do deal with the spark plugs, so haven't noticed this behaviour on other vehicles. But isn't the whole point in the insulation on the cables to prevent the current from escaping?
I'm pretty sure that the cables are the original stock cables, especially since the previous owner don't seem to have really maintained the car in any way. They're in very good condition though, with no visible wear on them. The distributor don't have any visible cracks, but I haven't yet gotten to check the inside condition of it. Have no idea what condition the coil is in, because it's buried so deep in the engine room that I can't get to it.
Anyhows, I don't need anyone pointing out how stupid it is to touch high-voltage curcuitry while it's live. I just want to know if it's normal for the cables to deliver current through the insulation or if I need to look for faulty components.