•Wash your hands thoroughly with soap and water, then dry them.
•Put the PowerBook or iBook into sleep, then wake it up.
•Hold your finger in one corner of the pad, and tap your other finger in the opposite corner.
•Clean the trackpad with a little bit of soap and water, drying thoroughly.
•Do a PRAM reset. (Hold down Command-Option-P-R while restarting, do it until you've heard the chimes 3 times.)
•Try resetting the PMU.
Here are a couple of links to some Apple Knowledge Base articles about jumpy trackpads:
•PowerBook: Jumpy or Erratic Trackpad Operation
•PowerBook and iBook: Trackpad Does Not Respond
Thanks to Mauricio9 , rjackson , and mgilliland for their suggestions.