On the Linksys WRT54G with openwrt firmware, I found  there is a through path  of the  traffic between the wireless clients, 
this traffic  operation  is before the bridge part of OS kernel . but I can not find where is this through path.
Is this locate at OS kernel ? or locate at the AP driver part  from  the boardcom chipset, is  only  the  binary verion  available for the  AP part of the boardcom ?

thanks

The wireless driver enables you to "isolate" clients by setting the variable wl0_ap_isolate to 1 :

nvrma set wl0_ap_isolate=1

this will prevent the clients from communicating each other directly.