A griddle I've had recently appears to have adopted a strange behavior -- It's flat when cold, but after being on the stove for a little while it bows upward away from the burner. This makes doing much with it nearly impossible -- At this point is it pretty much 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.