priceline and hotwire worked this way but it is a pain in the ass and you are not in control of when you come/go and when all is said and done, the fares are not that much better. There are so many other sites where you can get airfares for cheaper and control the time you come/go.
Try AirTran. Go directly to their site. I just got roundtrip from Dallas/Ft Worth to Philadelphia for $239.60. True I do have to stop over in Atlanta, but it isn't a long one and I only had to purchase the tickets one week before.