Dallas-Love (Southwest's base of operations) currently isn't allowed under the Wright Amendment. That is supposed to change sometime in the near future (by 2015, if I remember). However, capacity constraints on the airport will dictate how much Southwest is able to expand. While Boston isn't inconceivable, it probably won't be at the top of the list once the Wright Amendment goes the way of the dodo. Dallas-DFW is served by American, Jetblue and Spirit, so competition already exists on the route.
I am, however, surprised that Jetblue hasn't already started Houston. Given United's monopoly on the route and the fact that you're connecting two of the largest cities in the country, it seems like it would be a good fit for Jetblue. If Southwest is serious about moderate expansion at Logan, this is would be a plum route for them to start.