... So then even when the difficulty is at retarded proportions, the productivity won't necessarily have to decrease?
When total network was 1GHash/sec a block took 10 minutes, same as now. The whole point of difficulty is to keep productivity the same.
Were blocks worth more than 50 BTC back then? I remember reading in that one thread about the guy who bought two pizzas for 10,000 BTC that he was generating "a few thousand" bitcoins per day.
The number of coins per block is cut in half every 210,000 blocks. We are currently around block 136,746 so it will drop to 25 coins per block in about 17 months from now.