I have one new appreciation for fiat currencies - they're designed to circulate with a steady rate of inflation. It seems there's a hesitation of spending bitcoins knowing if you just wait a day it will go up, so it's being treated like a precious metal rather than a new way of paying for things. Edit: Thanks for the correction, meant to say fiat currencies tend to 'inflate', not deflate.
I don't believe the argument that a deflationary currency, by itself, will make people not be willing to buy things. Consider a savings account - why would anyone take money out of their savings account to buy things? If all you need to do is keep it in the account, it will make more money, so why spend it?
BTC on the other hand, if we reach a steady deflationary state, beats the rate of deflation by definition, and just by you sitting on it. Slightly different situation.
--edit-- Also see here - http://eprint.iacr.org/2012/584.pdf It seems the ~80% of bitcoin are long-term dormant, so people are just holding on to them, regardless of the reason.