Are you sure he specifically said it wouldn't require any sort of tube? I was under the impression that Elon's Hyperloop would be a derivative of the concept of Evacuated Tube Transport - a capsule containing passengers launched inside a tube under vacuum (eliminating air resistance) and levitated using some sort of MagLev (eliminating rolling resistance).
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.
I don´t think is a vacuum tube, it seems more like a tube with railgun like propulsion(elevated via towers, like power lines, even using the same power lines routes), but the guiding is maybe based on aerodinamics. There must be some kind of sound contention because he says it is supersonic.
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.
The cannon idea is really interesting. Musk's initiatives are all about reducing the cost of long term operation in order to disrupt the market. Most of the operational cost of air transportation is (I believe) in the take-off. If you eliminated the need for the aircraft itself to takeoff by projecting it with a rail-gun then this could theoretically reduce costs greatly. Essentially creating airline bullet gliders. Just a theory.
I would love it if this were true. I have been saying (only semi-tongue-in-cheek) for years that getting shot out of a cannon would be a great form of transportation, if only the bugs could get worked out (i.e., eliminate possibility of mid-air collisions). :-)