Actually, we were always licensed under Expat, but no one noticed.

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".