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: 
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.
Raw, unsalted meat is completely kosher. Just rinse and eat. Shulchan Aruch Yoreh Deah siman 67 siff 2.

EDIT. The Shach in siman 76 s.k. 2 also point out that unsalted roasted meat even if only rare is also permitted to eat.
The point of my original answer was to dispel any misconceptions about blood, but this edit will answer the assumed question more directly as pointed out in the comments. 

One more point of clarification as pertaining to cooked, not roasted meat, is after salting the red juice is not considered blood,but called mohel, as pointed out by Monica. This is the Ashkenazi law lichatchila. For Sfardim however, if possible they should be putting the raw salted and washed meat into boiling, not warm or cold, water in order to seal in the red juice which the Rambam did in fact consider blood, and the Shulchan Aruch says to take into account, when possible. See siman 69 siff 19. This is process is called chalita. 

I don't know any Sfardim who actually do this, whether at home or in restaurants (though some Teimanim do). So for all Acheinu bnei Hamizrach please comment or quote any modern day rulings, but this is what the Shulchan Aruch says and some people reading this question and these answers might find this interesting.