WEP's weakness is there's only one key for every packet. If you have someone who downloads a lot, you can just turn your wireless card into capture mode and easily save enough frames in a day or two to crack the WEP without any kind of active attack. What's even worse is using words as passwords, which are usually hashed by MD5 into WEP keys. If you do that, you've made cracking the password about 1,000 times easier (at least).

