Well I wouldn't be running full contact drills. :p The field should be heated from underneath. If you remember the Oakland @ Denver game this year, it was in the middle of a blizzard and the field was kept pretty darn green most of the game. Although someone may have "forgetten" to flip the switch on. A lot of these fields have some pretty advanced (and cool) designs in them that allow excellent drainage in case of rain and internal heating from the cold.

Oh yeah, so much for the Eagles being rusty and their offense grinding to a halt w/o TO.