This exact same thing happened to me after I upgraded fromSnow Leopard (v10.6.8) to Lion 10.7.2.
My Snow Leopard-running MacBook had been connectingautomatically from sleep with my ATT WIRE2 router for 6 months. After upgradingto Lion, my preferred network wouldn’t connect without my reselecting of it inthe list of available networks even though in WAS already in my PreferredNetworks list. The network would connect successfully then with nothing more requiredfrom me.
Another Lion-running MacBook in my house DID loginautomatically to our router so the router was clearly not the problem.
When I compared the Preferred Network list between the twolaptops, the WIRE2 network name was identical but the security protocol was different.
The laptop that connected automatically showed WPA Personal securityprotocol and the laptop that failed every time (the laptop I’d recentlyupgraded from Snow Leopard) showed WEP security protocol.
By the way, my ATT 2WIRE router configuration paperworkdesignated WEP as the required security protocol for my router. (Yep thesuccessfully-connecting MacBook was apparently misconfigured.)
When I
---deleted the errant computer’s 2WIRE network from mypreferred list,
---clicked + to Add a new Wi-Fi network,
---used the same network name for a new preferred network
---but use the WPA Personal security protocol with the ATT2WIRE paperwork’s WEP password
the setup works.
When I close my newly upgraded Lion-running MacBook and itgoes to sleep, when I open it back up airport automatically connects to mywireless network every time.
The fix persists through a router reset and rebooting myMacBook.
I hope this helps someone.
Red Head