Voltage doesn't really matter, it's a requirement to get a certain voltage to pass through the skin, but voltage doesn't have any impact on "damage".
Current is what does damage.
I've heard tons of claims as to what will kill you. In EE school is was 60mA AC and ~100mA DC across your chest that would send your heart into fibrillation.
I've seen claims that < 10mA directly through your heart could do the same. Honestly both are probably correct. I don't know what a real electrical model of the body looks like, but I don't have a hard time believing that if there were 100mA running through my body from one hand to the other that only 10% would pass through my heart directly.
I've worked on live phone lines before (~58V DC with off hook) and that didn't pass through my skin initially. A half hour of being in the 105°F degree attic and sweaty hands later, it made my finger twitch and didn't feel good. On another occasion I was working on a phone line when someone dialed it... that sucked... the ring pulse is 120V AC (current limit though) and does not feel good at all.
It only takes a couple milliamps to seriously get your attention, 10+mA will lock up muscles, this is highly frequency-dependent though.
To get back to your point... greater than 100-200mA is when you'd expect to start to see flesh burning and things like that. But obviously from the heart discussion above, localized currents that are much smaller can be deadly.
I don't really know if there is a firm rule as to what's "safe". The current debate over the use of tasers, for example, would seem to indicate there isn't much conclusive evidence.