Second, in the hotspot scenarios where this is likely to be a problem (in IPv4 -or- IPv6) it's addressed by the "AP isolation" feature that's getting close to omnipresent even in the low end APs. With this feature enabled, stations are not allowed to talk to each other over the wlan; they can only talk to hosts on the wired side of the lan.
Not related to the original subject, neither to IPv6 usability on WLANs, just a small comment: As far as I understand "AP isolation" doesn't work if you don't have a WLAN controller but do have more than one APs. E.g. in the following setup ap1--sw1--sw2--ap2 with "AP isolation" turned on, clients associated to ap1 cannot communicate directly with other clients associated to ap1, however they can communicate directly with those associated to ap2. Broadcast from ap1's clients does also get to all clients at ap2. Regards, András