I have a late 2010 iMac. After upgrading to OS X Lion last year I started having issues with the WiFi reconnecting after I put the computer to sleep. I tried a lot of the suggestions I saw in forum posts to fix it but the problem persisted. I eventually sought out my router and ended up adjusting its DNS settings to use Google's Public DNS servers. My iMac was already using this but setting this on the router itself seems to have helped with my issues.
update The latest OS X Lion update has an official fix for this issue.