I'm having this issue as well. Still can't find a fix.
Guys, try this. Open your Network Preferences (in System Preferences) and click the Advanced button. Under the Airport tab, select each network and delete them until the list is empty. Get and write down any passwords or settings you need to if you don't have them handy before you do this.
Click Apply and then close System Preferences.
Go here: Macintosh HD>Library>Preferences
Delete these prefs:
com.apple.alf.plist
com.apple.nat.plist
com.apple.networkConfig.plist
Those are the files where your network prefs are stored. Technically you can get away with leaving the last two, but the alf.plist is the airport pref file so you want to delete that one. No worries, when you set up again this pref will be regenerated.
Restart. Reconnect to the network you intend to use and enter all the login details. Make sure the password is stored in the keychain. You'll have to do this for each network you use until you get all of them back in.
That should take care of it.
I had this problem a year or so ago and the Airport/Network preferences is the culprit. You have to delete everything out and start over after a restart.
Hope it helps.