Answer:
Bottom of cup marked recyclable "PS 6".
Type 6 - Polystyrene, styrene, polystyrene foam
Not heat stable. Do not use in microwave. Do not microwave food in a styrofoam container! In addition to not being heat stable, polystyrene is a potential human carcinogen and usually contains bisphenol A.
http://www.scn.org/~bk269/plastics.html