"OS X Lion: iMac may not automatically reconnect to Wi-Fi network after waking from sleep
OS X Lion, iMac (21.5-inch, Late 2009), iMac (21.5-inch, Mid 2010), iMac (21.5-inch, Mid 2011), iMac (27-inch, Late 2009), iMac (27-inch, Mid 2010), iMac (27-inch, Mid 2011)
After waking an iMac running OS X Lion v10.7.3 from sleep, the computer may not automatically reconnect to the Wi-Fi network it was associated with prior to sleep.
Download and install the iMac Wi-Fi Update, which resolves an issue that may cause an iMac to not automatically connect to a known Wi-Fi network after waking from sleep.
For additional help with Wi-Fi, please see:
Troubleshooting Wi-Fi issues in OS X Lion and Mac OS X v10.6.
When I first posted the above there was no Wi-Fi Update Download page yet, but there is now: