I guess it is just the price change then that messes it up for me, to where its not a bitcoin is worth a bitcoin like a dollar is worth a dollar. I know for so long ya know I can buy a candybar for a dollar, and probably get some change back depending on if its 1990 or 2011 but still pretty damn close, where months ago a bitcoin was nothing, then it was worth 30 bux, then hit 9 bux, then 12, now 15, stablizing in the 12 to 15 range now so its just a bitcoin can not buy the same thing today as it can say tomorrow or next week. Thats the part that messes me up I guess, is how quick the value is changing daily.
In reality everything floats to everything. There is not an absolute. What happens is that for commodity we use the most liquid asset as a reference, but in reality you can use anything as reference. The most liquid asset is that one that everybody accepts, in the USA the dollar, in the EU the euro, ... As JoelKatz explained, when you have a big base of people using bitcoins it will happen the same with bitcoins.
And btw, from 1990 from 2011 the prices in dollars have gone up a lot.