Found another Sniffer which might do the job,

Kismet is an 802.11 wireless network sniffer - this is different from a normal network sniffer (such as Ethereal or tcpdump) because it separates and identifies different wireless networks in the area. Kismet works with any 802.11b wireless card which is capable of reporting raw packets (rfmon support), which include any prism2 based card (Linksys, D-Link, Rangelan, etc), Cisco Aironet cards, and Orinoco based cards. Kismet also supports the WSP100 802.11b remote sensor by Network Chemistry and is able to monitor 802.11a networks with cards which use the ar5k chipset.

http://www.kismetwireless.net/

They do give a Faq on this problem at the ethereal website ,did u have a look there??

scroll down to Q 5.20

http://www.ethereal.com/faq.html

Sorry if its not much help,