Apple recently released the iMac Wi-Fi Update v1.0, an update that resolves an issue that could cause iMacs to not automatically connect to known Wi-Fi networks after returning from sleep mode.
All owners of iMacs released in late 2009 (or newer) with OS X 10.7.3 must install the update, which weighs 25.81MB.
More details can be obtained in this support article.
