If you're like me, you use multiple pointing devices with your Macbook. I have several and depending on what I'm working on, sometimes it makes sense to disable the built-in trackpad on your Macbook while using other devices. With Lion (OS X 10.7), is very simple.
Open up System Preferences and select Universal Access...
Under Universal Access, select 'Mouse and Trackpad', and then select 'Trackpad Options...'
Within 'Trackpad Options...', select 'Ignore built-in trackpad when mouse or wireless trackpad is present'.
Select 'Done' and you're all set.