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.
"Silent enim leges inter arma." -- Cicero