I have found this chart interesting.

difficulty by itself really has no meaning for a miner.
USD:BTC by itself likewise really has no meaning.

What matter is price & difficulty.   
If price of BTC falls 90% and difficulty falls 90% I will make the same amount (in USD).
Likewise if price of BTC rises 500% and difficulty rises 500% I will still make the same amount (in USD).

Till now I haven't seen a good way to express that important relationship.

http://bitcoinx.com/charts/

This chart illustrates it well (showing daily revenue in dollars per 100MHash/s).
http://bitcoinx.com/charts/chart_large_lin_30d.png

Long term we would expect this trend to fall continually because the cost to produce 100MHash/s rig continually falls (faster and cheaper GPU due to Moore's law).


Great chart by the author (although I think changing the unit to GH/s might be more applicable today).