The miners are responding to the rise in the market price of Bitcoin. If the price stalls, so will the rise in difficulty. If the price falls, the difficulty will slowly drop off to meet it. Changes in difficulty tend to follow changes in price with a 6 week lag.
Check this out: http://forum.bitcoin.org/index.php?topic=7427.0
It seems as though the price per BTC in USD tends to follow the difficulty with an upward trend... Granted, the most recent difficulty changes (ie, past 30 days) have been pretty steep, the price seems to have a higher rate of trend than the difficulty in these peak times, so I'll mine until the cost outweighs the benefits, or the value of the hardware changes.