To be announced tomorrow, Tuesday August 5th according to Scott McCartney's Middle Seat Column in the Wall Street Journal (Subscription required), Delta and Gogo will announce that Delta, using Gogo's internet Wi-FI technology will offer inflight Wi-Fi access throughout its entire domestic fleet. This starts on 75 MD 80s later this fall and will end when the entire domestic fleet of 300+ aircraft are fitted by next summer.

This is the same service AA is using and recently conducted tests on. See discussion in the AA forum. You can jump to the discussion about Gogo here. Like AA, DL and GoGo will prohibit voice calls using VoIP technoliogy.

More information on Gogo itself is located here. They have taken down the registration part. I'm not sure if you signed up during the AA test, the information is still there.