Apple rolled out Wi-Fi Update 1.0 for iMac owners late last Friday. The update for late 2009 and newer iMacs addressed an issue where the computer doesn’t automatically reconnect to wireless networks after waking from sleep when running OS X Lion.
Wi-Fi gets a fix with iMac update for Lion users
The update requires OS X 10.7.3 and is available via the Software Update application on supported iMacs. It’s also available for download through the Apple Support website.