Actually, this leads me to ask a second question: what is the general defense to economists' general argument against deflation as it relates to bitcoin. How are people incentivized to spend, rather than hoard bitcoins?
It's pretty analogous to gold. Gold & silver used to be valid currencies, then gold-backed and silver-backed currencies, and now plain fiat currencies are the method of daily exchange.
So in short, people are not incentivized, which is why economists on balance don't believe it is a currency that is worth a damn.
The usual quote that goes along with this is Gresham's Law:
http://en.wikipedia.org/wiki/Gresham's_law