My thoughts on mining and difficulty are that the difficulty will always settle at a point that is relative to the amount of money people are willing to spend on the widest available production method.

If the most efficient (and widely available) hardware you can buy is 5Mh/s/dollar, then the majority of miners will use that hardware and the network hashrate will rise accordingly. This means, that unless you are on the leading edge with a new piece of hardware, you can never really gain much advantage.

For example, if Bob the statistically average miner maintains $1000 worth of mining hardware, and upgrades at a rate that is also average, he will never really progress in terms of his share of the entire network hashrate, which means he will always (again, on average) generate bit coins at a constant rate, assuming he is keeping up with the available technology.

Then again, I am a newb, so what do I know.