How can one look up the tail number for a plane currently in flight? (Even just being able to do this with US airspace would be great.)

I'm sure there's a way to do this and I can't figure it out. Sites like http://ftdashboard.net/overview/baflights.htm are GREAT for tracking BA equipment and trends, but they're also after the fact.

I imagine there is a way to determine actual tail numbers for planes in-flight, right? Most often I imagine that I'd use this to figure out if I'm going to be on a new First plane when not originating in London.. for example, if you know a plane does a turn-around at the destination, you could figure it out ahead of time pretty easily! thanks for any tips!