I struggled for a while to get my wireless card work after installing / upgrading Ubuntu on my T400 Thinkpad.

My T400 came with wireless card : Realtek Semiconductor .

To get it working, I followed these steps :

1- Open nautilus

sudo nautilus

2- Browse to /lib/modules/

3- Search for any rtl8192se and remove them

4-Download the drivers from Realtek website (Search for RTL8192SE)
Click to go to Realtek Website

5- Install the drivers

sudo apt-get install build-essential linux-source-2.6.32
tar -xzvf rtl8192se_linux_2.6.0015.0127.2010.tar.gz
cd rtl8192se_linux_2.6.0015.0127.2010
sudo make
sudo su
make install
modprobe r8192se_pci

6- Reboot and it should be working out of the box.