I think the coldest I've played in was -10 F (not wind chill). Bearable in 3+ layers, if it's sunny and the wind isn't too bad. Just make sure your hand isn't wet when you reach into those metal chains or basket to grab your disc.
Shoot...it's forecasted to be 21 for a high here tomorrow afternoon, and lots of players are asking each other where they're playing, since it'll be a "heat wave."
A friend of mine played at an Ice Bowl in St. Cloud, MN several years ago, where I think it was -12 to -15 F (not wind chill). Threw a drive that hit a tree, and his disc split in half. Now THAT'S cold.