Personally i like the card and have only one question.
Why not make it work like Fire Shield? You could then synergise with the Ice Shield making it quite an interesting combo. If you were to do this you proably want to raise the cost of the card though and lower the attack.
Maybe:
Frostbite:
Cost: 6
Damage: 3
Ability: Inflicts 1 damage to every frozen Creature at end of turn
Frostbite (Upped)
Cost: 6
Damage: 4
Ability: Inflicts 1 damage to every frozen Creature at end of turn
The other suggestion in this was to make the damage at the start of your turn thus providing better oppurtunity for creature control.
Just ideas but I like your work.
Scyther