Comment 20 for bug 496640

Had this issue too. Maybe this sounds stupid, but could you try this:
- Click left on the network manager symbol in the system tray.
- Check enable networking

Solved it for me, though I'm not sure if anything else I did before finding the option is required. If it doesn't work, try the only other thing I did:
sudo rmmod iwlagn
sudo modprobe iwlagn