I have a Lenovo Thinkpad X230T, and I previously had a Lenovo Thinkpad X60T. My current machine (the 230T) runs 12.04, whereas my old X60T ran 10.04. They both had this issue connecting to two wireless networks at my school, while no one else I know running linux has had this issue. Basically, one of two things happens. First, I often can connect to the wireless network, according to the network manager, and am given an ip address, but then actually have no network connection (ie, I can't ping anything) despite the network manager reporting some nonzero network speed. Another (presumably related issue) is that occasionally I will see a massive spike in network activity after which the network manager still thinks I'm connected but I get no network connection (a reported speed of 0 and inability to ping anything)
My laptop has an Intel Centrino Ultimate N wireless card, and the drivers appear to be working. This problem is not seen by a lot of other people running Ubuntu at my school. Any help would be greatly appreciated.