I'm new to this board, but I'm sure you all have discussed the space elevator concept. I'm convinced that will be our avenue to cheap access to space. I believe the only breakthrough required to implement a space elevator is the development of cable material with suitable qualities. I'm not a material scientist, but from what I've read there are no known show stoppers to the eventual use of carbon nanotubes for this purpose. Still, we are several orders of magnitude away from the cable requirements in terms of cost and properties. My own lay WAG is that we will be able to produce the required cable in a 15-30 yr time frame. Given the time it takes to develop, test, and refine rocket designs, I really don't think fleets of rockets will ever come to fruition as a way to get to LEO. I just don't think they will be able to compete with a space elevator. Now, once you are at the top of the space elevator, that's where you can start having fun with rockets. Solar system travel above the gravity wells will require much different designs than those digging their way from inside the wells.

Of course, if there is a problem with development of the cable in a reasonable time frame, then maybe there will be some interesting rocket developments (e.g. ramjets, laser assists) to lower the cost to orbit.