The guideline I grew up with was "it's done when there's no more pink. If there's pink in the meat, that means it didn't cook all the way through, and you're apt to get E. coli and die if you eat it."
Hygenic commentary aside, I still very much prefer thoroughly cooked meat. Hal's tried to get me to at least order medium sometimes but I remain unconvinced. >_>
E. coli contaminates the outside of the steak due to unclean butchering procedures. As long as the outside is seared, you can't get it, even if the inside is raw and bloody. I... imagine there's something that can live in the muscle itself that might pass on to humans, but that'd be about a million times more rare than e. coli.
Ground beef, on the other hand, does need to be plenty hot the whole way through, because everything is being mashed together, so any contaminated surface can get the whole batch.