Tom Goldstein calls the same-sex marriage cases being considered today "the most significant cases these nine Justices have ever considered, and probably that they will ever decide":
I have never before seen cases that I believed would be discussed two hundred years from now. Bush v. Gore and Obamacare were relative pipsqueaks. The government’s assertion of the power to prohibit a loving couple to marry, or to refuse to recognize such a marriage, is profound. So is the opposite claim that five Justices can read the federal Constitution to strip the people of the power to enact the laws governing such a foundational social institution.
SCOTUS could announce which cases it is taking this afternoon or on Monday morning.