"FlightAware is proud to release a major upgrade to airline flight tracking on FlightAware.com. We have been working behind the scenes for several months to enhance our flight tracking services by adding many new features that are important to our users that track airline flights. When using FlightAware.com to track airline flights, you will now receive international coverage, gate information, as well as more timely, accurate, and relevant information."

Coming Summer 2010
The ability to track an airliner by the registration number. For supported airlines, you'll be able to type in the registration (e.g., N12345) and track that individual airplane, even if it flies as an airline flight identifier.

For more, see:

This will be very cool!
Track down the aircraft tail number down to the FlightID on FlightAware.!