Last week, I joined the ranks of those having problems with WiFi using Leopard, my MacBook Pro and Apple's Airport Extreme. On the advice of users on Apple's forums I turned off IPv6 networking. So far, it seems to be working.
Now, a week's worth of trouble-free networking does not a fix make. And yet, since making that change, I've had solid, steady wireless connections. So if you're having similar problems, I can at least tell you that the change did no harm. YMMV.