You'll have to run mixed mode (or whatever the AP you buy calls it). If you run .11g only he will not be able to associate to it. Well I've never tried it, it might associate as that is basically beacons, but no data will be passed because it won't be talking CCK, which is what the .11b client will be using. Mixed mode will degrade your available bandwidth though how much depends on the what product your talking about. They vary a bit.