Apple has released a small patch to fix a problem a number of iMac users have been experiencing with network connectivity on their systems after waking from sleep. Apparently, after the latest OS X 10.7.3 update, people with iMac systems from late 2009 or newer found that the system wouldn't connect to their preferred wireless network, requiring the user to manually select the network from the Wi-Fi menu.
While not a serious bug, it was a problem that caused a bit of frustration with users. MacFixIt readers wrote in asking about this issue, and while some have had success implementing my script-based workaround that automatically detects the system's Wi-Fi status and triggers it to connect, others have preferred to have the system behave as it ought to.
Apple recently released a small update that addresses this issue by replacing a faulty driver that shipped with OS X. The update is a 26MB download (even though the updated files are only a few MB in size) that should be available in Software Update for affected systems, so be sure to check there first (Software Update is available in the Apple menu). Alternatively, you can download the update from the iMac W-Fi Update v1.0 Web page. The update will require OS X 10.7.3 to install.
Keep in mind that this update is only for the iMac systems, and will not work in other systems experiencing similar problems. In these cases you can try some more general troubleshooting steps or try the script-based workaround mentioned above.