For the security wonks, Ethereal 0.10.9 now detects and flags weak initialization vectors (IVs) for WEP keys: An initialization vector (IV) is an attempt to increase randomness in a publicly available encryption stream. Combine a truly random IV from a large number space with a key set by a user and each packet has additional protection against brute force attacks. WEP was weak to begin with because the IV space was too small, forcing reuse.
Errors in implementation meant that IVs are rapidly reused on some networks. A flaw in the encryption algorithm further means that certain IVs, called weak IVs, reveal more information about the secret part of the WEP key than others--about 9,000 weak IVs out of 16,000,000 possible ones. (WPA, by the way, has a 48-bit IV instead of a 24-bit one without the weakness problem.)
The weak IVs are much more interesting than others because they have a statistically higher likelihood of resulting in a crack. Gathering weak IVs quickly produces a crack faster than gathering lots of strong IVs. (Some WEP-generating Wi-Fi adapters exclude weak IVs from their IV creation process as part of firmware design.) Gather a few thousand weak IVs, and you have a chance at cracking the key. On busy networks, this might be a few minutes using the latest cracking software.
Ethereal 0.10.9 detects these weak IVs, which either means you know you can crack a WEP key quickly or you know that your network has weak protection and ought to be upgraded. [link via Nigel Ballard; thanks to him and Jim Thompson for reality checks.]