I've been having this problem since yesterday. My laptop (DEll Inspiron N5010) was connected to my Wi-Fi router (TEW 651 BR) and worked properly. However, after I did a few updates to Xubuntu and I rebooted my laptop it didn't connect to my router anymore. I tried to connect to my neighbours's routers and it worked.
After resetting the router, it seems the router has saved my laptop's MAC address, but without actually connecting to it.
I think it might be a kernel bug. But I'm not sure.
NOTE: I haven't changed any of my router settings since then. All other devices from my house connect properly to the router.