802.11b devices apparently don’t slow down AirPort Extreme Base stations according to this article:
Mixing clients on an AirPort Extreme network
When you mix 802.11b (AirPort) and 802.11g (AirPort Extreme) clients on an AirPort Extreme network, each type of client receives an appropriate data throughput rate. The 802.11g clients continue to receive data at a higher rate than 802.11b clients.
