From the US website, "Gogo Inflight Internet has arrived on all of our Airbus A321 aircraft."

http://www.usairways.com/en-US/trave...eair/wifi.html

However, a change in aircraft due to maintenance or other problems is always a possibility, and the A321s are the only US planes with in-flight WiFi. I expect US would try to avoid such a change, but sometimes it's unavoidable. There also exists the possibility of maintenance issues with the WiFi system itself, and I don't believe such problems would prevent the plane from being used for the flight.

Thus, while there's a high probability of WiFi being available on an A321 scheduled flight, you cannot consider such service to be guaranteed.