I've noticed when running airodump, a couple of probes that show an ESSID length = 0. I was wondering if they are using the essid of "ANY". They seem to associate with an AP on channel 6, but at times I've seen them on channel 1, and as far as I know my AP is the only one on that channel. If that's the case, it appears to be a good way to keep your essid hidden.