The FBI has publicly demonstrated braking a 128 bit WEP key in about three minutes. What was once thought to take many hours of work, can now be broken more or less at will.
The purpose was to enlighten business on the potential problems they might (will?) suffer from using wireless networks.
click here for the report and an overview of how they did it. Obviously the details are sparse, as it would be foolish to detail the whole process. But I would suspect anyone with a good wireless network knowledge, could work the whole scenario out without too much trouble.
There is also some sound advice at the end of the report, on how to make your wireless network a little more secure.