To look at it the other way, the value of the bitcoin has not changed. Its price increase in dollar is just a reflection of the inflationary nature of the dollar.
But that's obviously not true. The value of bitcoins has changed wildly, independent of inflation in the dollar.
For instance, the dollar hasn't lost half it's value over the past year. Your rent hasn't doubled. Your grocery bill hasn't doubled. Your salary hasn't doubled. But the value of a bitcoin has: http://bitcoincharts.com/charts/mtgoxUSD#rg360ztgSzm1g10zm2g...
