It's been moved to System Preferences -> Universal Access on my BlackBook. About 2/3 down.
Nick
Brussels
Go to the Universal Access preference pane and click on the Mouse and Trackpad section. Then click Trackpad options near the bottom of the pane. One of the options that pops up is Ignore built in trackpad. Click that one.
What mouse do you have?
Most mice have a switch on the underside you can flick on or off.