I wonder how long until these systems become commonplace. The higher fuel prices go, the more economical it would be to add these systems. Just think of all the fuel burned while jets are taxiing or waiting in line on the taxiway. Not to mention the reduction in FOD and jetblast issues during taxi.
It is a very good idea, BUT the only issue that I could forsee, what if the plane gets all the way out to the runway to only find out that an engine will not start? Yes, it could just taxi back to the gate under the power of the electric motors, but would they catch crap from the airport if it becomes too common? Just a random thought that came to me.