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.
Most of the other answers seem to be working under the assumption that the OP is basing the question on the prohibition against consuming blood (although this motivation is not stated in the question). Taking the question at its face value, however, the OP just wants to know how well meat needs to be cooked in order to be Kosher. As YDK's answer suggests (albeit somewhat vaguely), cooking has nothing to do with the Kashering process. Yes, we could get technical and discuss roasting meat in lieu of salting as a means of blood-removal, etc., but if the question is taken at face value, it is based on a faulty premise, probably due to a misunderstanding, that meat needs to be cooked 'X' amount in order to be Kosher. It does not.