I don't believe that was it. I've seen my Airport speed drop due to weak signal and I never had that option selected. I think it happens automatically when the signal gets weak. Perhaps whatever that option did occurs automatically in Leopard now.
I know on one of my computer at least, the Mac mini, Airport is much improved under Leopard and no longer drops out like it did under Tiger.
Well, then your mini is better than my MBP, which has constant dropouts like it did after the airport update that came after 10.4.10 and there's no way to roll back to the 10.4.10 driver like I did with Tiger.