It's on US Airways A321 planes. I used it on a flight from, I think, PHL->LAX. Worked great. Was well worth the $10 they charge when you have a 6hr flight, but even better for free. Wish I could use this more, but there are only a small percentage of flights that have it onboard.
Just used this service on a Delta flight the other day, so it's not just AA. From the site:
all domestic AirTran Airways and Virgin America flights and on select Air Canada, Alaska Airlines, American Airlines, Delta Air Lines, Frontier Airlines, United Airlines and US Airways flights