Apple just released an update for all iMacs, Late 2009 or newer. It requires OS X Lion 10.7.3.
Weighing in at 25.81MB, the update can be downloaded from Apple's software update page.About iMac Wi-Fi Update v1.0
This update resolves an issue that may cause an iMac to not automatically connect to a known Wi-Fi network after waking from sleep.
It is recommended for all iMac (Late 2009 or newer) users running 10.7.3.
Article Link: Apple Releases iMac Wi-Fi Update v1.0