Yeast can survive in temperatures up to about 100ºF (38ºC). Anything higher will kill them. Ale yeast will go dormant if it gets into the lower 50s (11-12ºC), while lager yeast can go all the way into the upper 30s (2-3ºC) before going dormant. As long as you pitch the yeast into the wort after it has cooled to room temperature, you're fine.