According to this post on Boing Boing, FBI teams successfully cracked WEP (WiFi encryption) in 3 minutes using the following 2 methods:
The FBI team used the deauth feature of void11 to repeatedly disassociate the laptop from the access point. Desired additional traffic was then generated as Windows XP tried to re-associate back to the AP. Note that this is not a particularly stealthy attack, as the laptop user will notice a series of "Wireless Network unavailable" notifications in the taskbar of their desktop screen.
Another attack method the FBI team used is a replay attack. The basic premise of this attack is to capture at least one packet traveling from the victim laptop to victim access point. This packet can then be replayed into the network, causing the target AP to respond and provide more traffic to capture.
You still have to be fairly skilled technically to crack WEP and it’s definitely more secure than just leaving your network wide-open with no protection at all (i.e., don’t take this as a good reason not to enable security on your WiFi). Seriously, the number of wireless access points that I can find in my neighborhood with no security at all is staggering.