I think it's 15 milliamps
I thought they said 6 milliamps, but I've also heard that its 100 milliamps will stop you heart... I'm not sure though.
Human skin has a relatively high resistance, so my understanding is that 12v is not high enough to allow a full flow of current and therefore is relatively harmless.
Now if your skin is wet or you pierce your skin as this guy did
http://www.darwinawards.com/darwin/darwin1999-50.html, then the resistance would be extremely low and allow a lot of current to flow.
(not sure if that made since).
I'm sure Jimmy could answer correctly
Edit: damn it singularity beat me, but what he said is right.