The concept isn't entirely new but having someone with Musk's credibility and resources as a proponent may be the catalyst required to make it happen.
I distinctly recall seeing some PR material circling the internets a few months back from a consortium known as ET3.com The group claims to have been working on a similar project for the past decade.
Think "cheap". The cheapest vacuum is right above us: at 35 km up there's only 1% the air pressure at the surface. It takes some energy to get there, but if you're on a rail you can recoup it on the way down.
He also said that it would be energetically independent so he may be using the external part of the tube to install solar cells, and the tube itself may be used for accumulation (capacitation maybe?).It doesn´t need to be air tight, just somehow sound insulated. The steering or "levitation" inside the tube may be obtained via shock waves (note his actual experience with high energy aerodinamics due to the development in spaceX).
He surely is taking advantage of the knowledge gained in the R&D of SpaceX, Tesla and Solar city.
"it would cost you much less than an air ticket or car – much less than any other mode of transit – because the fundamental energy cost is so much lower" – https://www.youtube.com/watch?v=uegOUmgKB4E#t=46m03s
Oh, and he said elsewhere that you wouldn't have to wait, which implies individual cars. That means much more frontal area per person than a regular plane.
My money is on this: http://news.ycombinator.com/item?id=4806350