Skillet warps when hot -- Is it toast?
It is not toast if it gets flat afterwards. It is just too large for the burner on which you are using it. 

The general rule is: if a skillet wraps with the center bending up, re-bend it (if possible - coated ones can crack the coating) and use on a larger burner. If it wraps center-down, rebend and use on a smaller burner.