Late 2006 iMac with Lion I have this issue too. Using Airport Extreme 3rd Gen with 7.5.2 firmware. Connecting over 5Ghz n.
After wake from sleep WiFi connects fine, but DNS doesn't work. I can browse via IP fine. Restarting WiFi fixes things.
Tried everything mentioned here, including:
Resetting SMC.
Removing Bluetooth PAN.
Removing and re-adding network settings.
Fixing IP.
Adding extra DNS servers.
Deleting plists and re-setting up network.
Moving order of services to promote WiFi.
Switching off TimeMachine.
Fixing Location / Adding new location.
Removing all preferred networks.
Turning round three times while saying the name of the Scottish Play.
Cannot change beacon interval as Airport Extreme doesn't allow that.
Never had any problems with Tiger/Leopard/Snow Leopard. Early MacBook Air runnung Lion connecting fine after sleep.
I think what we have here is a bug. I wonder if Apple will spot/fix it?