Apple has issued an update over the weekend specifically for iMacs (late 2009 and newer) entitled Wi-Fi Update v1.0 that is said to correct an issue where the unit does not automatically re-connect to a known Wi-Fi network after waking from sleep. The update requires that users be running the latest version of Lion (10.7.3) and weighs in at 25.81MB. It's available for all supported OS X languages.
