You should probably separate out 3 different aspects of the questions- Musk's personal desires, the technical feasibility, and the political/economic feasibility.
Elon Musk has said that he has no interest in creating a working hyperloop himself, and therefore made the design open source. This strikes me as an uninteresting question.
The technical issues are fairly likely to be more-or-less settled within the next year.
The prediction they're interested in, I think, is whether anybody will create a working hyperloop. Or, possibly, anybody in California- p(China, Japan, the EU, or India create a working hyperloop by 2100|the basic ideas behind the hyperloop are sound and there is no global catastrophe) strikes me as uncontroversially close to one.
Close to one? So the probability that some cheaper/better alternative that we're not considering is developed before anyone gets around to building the hyperloop is close to zero? That doesn't look uncontroversial to me.
I was studying in the LW Study Hall, and during our break someone posted this link to the official hyperloop announcement:
http://www.spacex.com/sites/spacex/files/hyperloop_alpha-20130812.pdf
One member was doubtful it would get past regulations, and another said "tentative p>0.05 that a hyperloop gets made by 2100", which was met with "p>0.05 that uploading people and moving them between bodies will be available by 2100".
It struck me that people might be interested in betting on things like this, or at least having a conversation about it.
A few predictions to start: