Apple has posted Wi-Fi Update 1.0 for for late 2009 and newer iMacs. You can obtain it via the Software Update component of Mac OS X’s Systems Preferences app.
Wi-Fi Update 1.0 fixes a problem where the iMac doesn’t automatically reconnect to wireless networks after waking from sleep when running OS X 10.7.x (“Lion”). The update requires OS X 10.7.3 and is 25.81MB.