I'm new to wireless but ...(o'reilly book very good and I think they produce a wireless hacks book too)
My understanding of WEP cracking is that you need multiple captures of the associations and transferred packets to then run a dictionary attack against the captured packets for the wep keys. WEP 128 would require several hundred thousand (maybe a million) packets captured making it very time consuming - the recommendation for protection against WEP cracking is to change the WEP key frequently (every 15minutes I think I read).