Upon upgrading Ubuntu 11.04 through 12.04 on a Dell System, the touchpad doesn't respond until the power plug is unplugged. Any idea why? To better clarify the behaviour of the touchpad: The touchpad moves in slow motion when the power is plugged otherwise, it works perfectly.
My guess is that there might actually be something wrong with the power supply as suggested in this clip from Wikipedia:
If the computer is powered by an external power supply (PSU), the detailed construction of the PSU will influence the virtual ground effect; a touchpad may work properly with one PSU but be jerky or malfunction with another (this does not imply any electrical risk whatsoever, a delicate capacitative ground, not a contact ground, is at issue). This has been known to cause touchpad problems when a manufacturer's PSU, which will have been designed to work with the touchpad, is replaced by a different type. This effect can be checked by touching a metallic part of the computer with the other hand and seeing if operation is restored. In some cases touching the (insulated) power supply with some part of the body, or using the computer on the lap instead of on a desk, while working can restore correct operation.
Maybe try another charger and see if the problem resolves.
As I said here and some other people say, a fast workaround is to touch notebook ground: Make a couple of loops around power plug with a clip and touch it while using the touchpad. You can also use any USB cable, just plug it and touch the other end, the metal shield (grounded). You can buy or make a DIY anti-static grounding wrist strap connected to the notebook ground (USB or clip/power plug), like Roreru's answer.
It's a ground loop problem with Synaptics touchpad (Dell Inspiron 14 in my case), maybe related to a bad power supply. I'll edit this reply if other PSU fix the problem.