I don't know enough about economics to judge, but is this growth sustainable and how high can it go?
Whether or not bitcoins are currently in a commodity bubble or not is speculation, but bitcoins are no more a currency than Iron, Gold or Tulips are.
In theory commodities can go to very high heights indeed.
Very interesting read[1] nonetheless.
[1] http://it.slashdot.org/comments.pl?sid=2875465&cid=40120...