Some auto tranny fluids will start to flash in the low-to-mid 300°F range, typically most by 350°F. Synthetics usually are safe above 400°F. ATF datasheets spec viscosity at 100°C. I guess the warning light really is a warning light, so if it comes on you are taking a ride in the danger zone and like Steve says, flush and fill would be wise because you might be cooking the lube.
'91 Pickup - Imelda
'08 Tacoma TRD - Donna
'09 Kawasaki KLR650
'12 Gunnar Rockhound 29
"Our task must be to free ourselves from this prison by widening our circle of compassion to embrace all living creatures and the whole of nature in its beauty. Nobody is able to achieve this completely, but the striving for such achievement is in itself a part of the liberation and a foundation for inner security. " --Albert Einstein