Last year, there was a Wakefield/Wi-Fi Alliance study that found 76% of travelers would choose an airline based on wi-fi availability. (http://www.wi-fi.org/news_articles.php?f=media_news&news...) A number which, I can only imagine, has been increasing over time. And, as we've seen (re: Starbucks) there's a big difference between wi-fi and free wi-fi. As far as I can tell skimming a couple searches, there doesn't seem to be any line that offers free wifi... yet.
It seems like there's a coming sea-change, however. The going theory is that free wi-fi is going to be a standard check-box item by the middle of next year - with many assuming Virgin and SWA will likely be leading the way.
On a related note, I fly pretty much exclusively Southwest. I took my first flight at just over a week old and have flown pretty regularly ever since. About 6-7 years ago, I made the change to flying SWA whenever possible and haven't looked back. Last time I flew to SF, it took two different flights with a transfer in between - and I got a free drink on both.