According to Wired, some spyware-related documents released by the FBI last week contained an interesting bit of information: In 2007 while working a case in Pittsburgh, the FBI's International Terrorism Operations Sections had a need to crack a wireless network. The FBI's Cryptographic and Electronic Analysis Unit (CEAU) was consulted, and "CEAU advised Pittsburgh that they could assist with a wireless hack to obtain a file tree, but not the hard drive content."
Wired doesn't report whether the FBI cracked a WEP or WPA encrypted signal.
More FBI Hacking: Feds Crack Wi-Fi to Gather Evidence