These stamps became available sometime back in March, but the first time I saw them was at the post office yesterday. I wanted to get a few, but they didn't have anymore. Luckily the lady behind the counter said they would be getting more soon.
The set includes works by Arshile Gorky, Adolph Gottlieb, Hans Hofmann, Wilhem de Kooning, Joan Mitchell, Robert Motherwell, Barnett Newman, Jackson Pollock, Mark Rothko, and Clyfford Still. USA Philatelic has a list of the paintings here.