I'm glad you asked. Basically, if the dollar ever becomes worthless, in other words if the value of the dollar is zero anything's, then any 1 thing should be worth an infinite number of dollars (even if there aren't that many dollars in circulation). So as we approach that zero-value-horizon of the dollar then we should begin to see the value of bitcoin (and basically everything to be honest) raising up inversely against the dollar. In that situation a cup of coffee would also cost an infinite number of dollars.
It might help to think of it as a different pair of currencies (since bitcoin is still so new, and everything is compared to the dollar these days), something like Euros and Yen perhaps. So if the Yen was ever someday worth zero Euros, then 1 Euro would be worth infinite Yen. And so would a cup of coffee, or some pants, or even anything that has any value at all. But just before the Yen reaches zero (assuming it takes a while), the Euro should show up in the trading charts as being worth an exponential number of Yen. This would happen as the Dollar or the Yen collapses down to a value of zero, not the ordinary trading days, but a total collapse.
However, nobody would be trading or shopping with Yen or the Dollar at that point so bitcoins, coffee, and everything else should still have a rational value in comparison to other currencies. It's just that in the case of something being worthless, everything else is priceless by comparison.
I hope this helps, but if I was wrong about anything then please let me know :)
RE: Why I believe that Bitcoins will be worth More Than 50 Million Dollars Each... Someday