It depends on how fast the deflation happens. BitCoin is not all that useful as a currency right now because it's value is so volatile. This is because it is subject to huge increases in demand as it spreads to more places across the globe. It is not hindered by any export controls or borders, which means it can "infect" whole nations/societies/cultures in lumps, as we saw recently with China, sparking a huge jump in price.
But eventually everyone will have heard of BitCoin, and I believe BitCoin's growth will slow down, and its curve will flatten and look more stable. The more people have BitCoin the less small groups of individuals, or even countries, can influence its price.
But look at the growth of vendors who are accepting BitCoin over the same period. For the same reason it doesn't make a whole lot of reason to spend BitCoin during this growth phase, it does make sense to accept BitCoins. And even though most merchants don't wish to speculate with their revenue, even keeping a "float" of currency in BitCoin could be profitable, and so it is reasonable that merchants and vendors might want to accept BitCoin even though not a whole lot people are spending them these days. That can only increase BitCoin's value as a currency in the long run.
If it was unwise or wise to spend something based on its changing value, no matter how slowly, then it would be rational for me to want to spend all my dollars for goods and services today because the dollar value is going down. But the US dollar isn't depreciating at a fast enough rate for it to be rational for me to spend all of my dollars today, right? This is because the currency is inflating at a pretty slow percentage rate. A slow inflation rate is not all that different from a slow deflation rate.
It all depends on what will ultimately be the rate of deflation of BitCoin. If it is sufficiently slow(1% annually, for instance) it will be useful as a currency. If it is all over the place, it won't be.