Question: If you have a steak medium to rare, such that it's still pink in the middle, is it kosher? At what point is meat cooked *enough*?

Answer: 
I'm assuming you are talking about steak which has been processed under rabbinical supervision.

Presently, in the US, with the exception of liver, all kosher meat is salted to remove problematic blood by the certifying agency before it hits the stores and restaurants. So meat which is edible does not have to be cooked "enough" and rare - or even steak tartare (raw) - is fine.
You don't say why you think a rare steak might be a problem. I've heard others ask this question based on the prohibition of eating blood -- "hey, if we're not supposed to eat blood, then what about steaks?". I'm answering on the assumption that this is your reason for asking.

Chabad [says](http://www.chabad.org/library/article_cdo/aid/682641/jewish/Is-a-medium-rare-steak-kosher.htm) that what's left in the meat after it's been kashered is juice, not blood, and ok. No sources cited, though I've heard this from a number of people (for what that's worth).