Well, at MK, almost all the stores along Main Street were open as were any stores which rides come out in were also open, even in Fantasyland. I noticed that the Dole Whip place in Adventureland, Mrs. Pott's in Fanstasyland and the ice cream place (the name of which is escaping me at the moment) in Tomorrowland was open as well. I didn't see any street vendors open or any of the big counter services places like Pecos Bill's or Cosmic Ray's open. That was the only park we were really in for extra hours as all the parks were open late when I there two weeks ago. However, riding Thunder Mountain at 2 in the morning was pretty cool
And at MGM, the only counter serve open was Backlot (until about 1/2 way through), then one of the stands near R&R. Writer's Stop was closed, Sid's was open.
We were at MK on 3/23 till about 2am... DH had a hamburger at Cosmic Ray's around 1am or so. They only had the burger section open - not the other two.