In Ubuntu 11.10, Dell Inspiron N4050 can’t disable its touchpad as usual way in System Settings. If you’re looking for a solution, try this method:

Open up a terminal window and execute command:

xinput list

It outputs a list of devices:

⎡ Virtual core pointer id=2 [master pointer (3)]
⎜ ↳ Virtual core XTEST pointer id=4 [slave pointer (2)]
⎜ ↳ USB OPTICAL MOUSE id=11 [slave pointer (2)]
⎜ ↳ ImPS/2 ALPS GlidePoint id=13 [slave pointer (2)]
⎣ Virtual core keyboard id=3 [master keyboard (2)]
↳ Virtual core XTEST keyboard id=5 [slave keyboard (3)]
↳ Power Button id=6 [slave keyboard (3)]
↳ Video Bus id=7 [slave keyboard (3)]
↳ Power Button id=8 [slave keyboard (3)]
↳ Sleep Button id=9 [slave keyboard (3)]
↳ Laptop_Integrated_Webcam_0.3M id=10 [slave keyboard (3)]
↳ AT Translated Set 2 keyboard id=12 [slave keyboard (3)]
↳ Dell WMI hotkeys id=14 [slave keyboard (3)]

The red line is the touchpad, its id is 13. So use this command to disable it:

xinput set-prop 13 "Device Enabled" 0

To re-enable the touchpad, run:

xinput set-prop 13 "Device Enabled" 1