If your laptop or netbook doesn't already have a key to toggle the touchpad on/off (or it doesn't work - as it seems there are issues with the Fn keys on some devices), you'll be glad to know that Touchpad Indicator now has an option to set a hotkey for quickly enabling / disabling the touchpad.
To set it, simply click the Touchpad Indicator, select Shortcut > Set Shortcut and then enter a letter (it will be used with Ctrl + ALT - you can't change those).
To install Touchpad Indicator in Ubuntu 10.10, use the following commands:
sudo add-apt-repository ppa:lorenzo-carbonell/atareao sudo apt-get update sudo apt-get install touchpad-indicator
Once installed, you'll find it under Applications > Accessories > Touchpad Indicator.
If you don't want to install Touchpad indicator, you can also do this with a script -> see the second part of this post: Ubuntu: Automatically Disable TouchPad While Typing