I dare to say that future is pretty far away. The T-shirt being sold at BTC2 and $50 in your example would be sold for a different amount of BTC if its $ market price would be $150, according to what $150 would mean in BTC by that time. Unless, the costs involved in producing that T-shirt would remain the same in BTC, which would imply paying everything with BTC, including salaries 
But that would mean the end of the $, wouldn't it?
But that would mean the end of the $, wouldn't it? I envisage a future scenario (that might not be more than a few years ahead) in which the purchasing power of the $ has been seriously eroded, even more than it has already. Bernanke has admitted to wanting to debase the $ a lot more to relieve the debt crisis, at least on that point he's come clean.
Investors in bitcoin are really more interested in its purchasing power, to which the $ exchange rate is a guide. What will we be able to buy with one bitcoin in future? It's not much good the exchange rate being over $100 if that amount is worth no more than $10 today. But if the BTC exchange rate becomes astronomically high, as in >>$1000 in today's money, then the exact exchange rate won't be as important anyway, I don't think, because there'll be less need to trade to fiat if BTC is already entrenched enough to justify that price. On a pro-rata basis this would not theoretically require anywhere near critical-mass adoption to achieve, but justifying a $1000 price and actually having one on Mt Gox or anywhere else are two different things, of course.