[jamesbrennan]

It isn't only iOS devices that do this. All operating systems probe for SSIDs that they have been previously associated with, thats what makes devies like the WiFi Pineapple possible [0].

[0] http://hakshop.myshopify.com/products/wifi-pineapple

[newman314]

However, Apple devices do do "optimization" to get on the network quickly. If you previously associated with a SSID, it'll get reconnected much faster.

See http://cafbit.com/entry/rapid_dhcp_or_how_do