Someone said this problem might arise in twenty or so years? Iīm afraid thatīs very optimistic! The big oil crisis is much much nearer than that! Weīre already on the brink.
For aviation I donīt really see any alternatives to oil based fuels. Hydrogen isnīt gonna work. It requires huge tanks and, like somebody said before, you need a lot of energy to even produce it! Getting oil out of the tar sands doesnīt make things much easier as well. That also takes an incredible amount of energy to extract!
So I think that, already in a few years time, flying will again only be possible for the wealthy and rich.
Read some more information on this site and the links provided there:
http://www.oildecline.com