Southwest Airlines announced today that beginning the first quarter of 2010 they would start offering Wi-Fi on flights. The service will be provided by Row44 which is an internet service provider. The current system has been in testing since February 2009 for such things as email and even streaming video for laptops, BlackBerry smartphones, etc.
Currently both Delta and American Airlines offer on-board Wi-Fi and recently US Airways announced they will also provide access in 2010.
Interestingly enough, Southwest did not however release any pricing information, but currently Delta charges $9.95 on flights of three hours or less, and $12.95 on flights of more than three hours and I would be very surprised if Southwest varied to far from this mark.