If people continue to obtain BTC at a pace that keeps up with 25% inflation and yet inflation drops to less than 15%, then this will give upward pressure (where now we see flat or upward pressure on price).
The thing that matters though is that it is already known when the per-block reward drop will occur (+/- a week or so) and the amount (from 50 BTC to 25 BTC). So that might explain why the BTC/USD is stable at $5 versus being $2.
Now what if December comes and the block reward drops and, surprise surprise, the BTC/USD exchange rate drops a little as well.
There aren't $50 million worth of Bitcoin commerce transactions occurring each day where 10.5 million BTC are needed (with "velocity" / turnover estimated at one transaction per-day.)
So the impact of the block reward could pretty much end up being a non-event.
It will all boil down to, are there uses for bitcoin that will start to gain momentum.
The block chain transaction level (even excluding SatoshiDICE) is growing at a fair clip and there are no reasons apparent why that trend should reverse (or slow down even).
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http://blockchain.info/charts/n-transactions-excluding-popular