I'm not sure if it will heat up the whole tank actually. I may end up putting the water heater in the water and sticking the heat mat to the side of the terrarium. Although, I've heard you CAN keep white's at room temperature as long as they're away from draughts so it may not be needed! I do mist my frogs, even though I manage to keep their tank fairly humid (theres condensation on the sides of the tank). I reckon that with a half aquatic tank, they shouldn't need misting as long as the water evaporates, which it should do if it's nice and warm Hope my theory works out ok...
And yup, frogs are FANTABULOUS pets! Not sure what I like best, rats or frogs. I went for frogs as their smell is less offensive!