Just found a site called FlightAware after reading some aviation newsgroups. It's a really slick site with a tie-in to the U.S. air traffic control system - it will tell you exactly where any flight under IFR conditions is located. You can track flights by tail number (for small aircraft) or by airline flight number.
Unfortunately it doesn't report on VFR flights, even if the flight is being tracked by ATC. So no help for me in the short term.