
This is actually real, the Mythbusters tested this myth and, at 10 degrees, your tounge will freeze to a metal pole upon contact.
8-)
Loooong before Mythbusters, I confirmed this theory myself, through careful experimentation. Field tests determined that not only flagpoles but pretty much any metal object left out in the cold will have this effect.

As for getting it unstuck, if you pull it off soon enough, it's not a big deal. Otherwise, cup your hands around your mouth (you are wearing gloves or mittens, I hope?) and just breathe heavily for a bit. Thinking about a lamp made of a plastic leg wearing a garter and fishnet stocking might help with this.

The warm air will eventually warm the metal enough to break the bond.
Another winter tip: exhalation can also be used to unfreeze stuck padlocks and car door locks.