Chris is right, it is a heat differential that can potentially cause glass to crack. If the glass is uniformly heated, at a reasonable rate, you shouldn't have any problem. Or if the temperature difference between a hot area and cold area isn't extreme, you shouldn't have a problem. Unfortunately, it is dependent on several factors, so there is no easy answer to let you know if you'll have a problem or not.

Believe it or not, thick glass can be more of a problem, since glass is also an insulator.