See http://en.wikipedia.org/wiki/MIT_License, but essentially MIT corresponds to several licenses, including the X11 license, however, our COPYING file contains the Expat (or most common MIT license)...
Justification for this being OK:
As Expat is strictly more restrictive, and things can be relicensed under a strictly more restrictive license, we can clear up the uncertainty with this commit without needing callbacks from everyone defined under "The Bitcoin Core Developers".