If the $/BTC goes up without difficulty going up then it makes it more profitable to buy/rent computing power in order to mine. But when people do that the difficulty will go up.
My guess is that [value of a coin]/difficulty will decrease over time because people will find the most efficient mining set ups, best chips, cheapest electricity, etc. So if you can't profitably mine right now, you probably never will. But other people will continually have better and cheaper methods so difficulty will continue to rise.