It's gotten over 100 here maybe once since I started commuting regularly. I just take it easy and keep hydrated. A lot I think would depend on your particular commute. I don't have to go far out of my way to incorporate a swim if it came down to that. I stop by a beach in the mornings to get in a swim once or twice a week during the summer anyway.

I met a person back in college that claimed to not like temps over 70 but I thought he was just some sort of anomaly or was out and out lying. To me low to mid 80's and low humidity are about ideal. As long as the humidity is low I don't mind 90 degree weather either, but it's rare that it gets that warm without the humidity climbing up there too.