I have a Dell Latitude, and on the system tray there is a little icon for the touch pad. Double clicking on that brings up a menu of items. One of the tabs is “Device Select” with which you can actually disable the touch pad and use the mouse stick (the thing between the G and H keys). It’s not a very precise solution, but if you almost have your mouse plugged in, it should help.
I thought there was a specific “Disable Touch Pad when Mouse is Present” option, but I can’t seem to find it.