Increase in difficulty does not make Bitcoin miners mine any harder, it just takes them a longer time to solve a block. Their power consumption per unit of time remains the same.
On the other hand, more miners joining the network and mining increases the difficulty. Correlation is similar, although causality is reversed.
So, what impact does the increased hashing speed (and thus increased difficulty) have on the ecology? Well, more resources (electricity, hardware) are used to create new coins. In return the Bitcoin network gets more and more secure from attacks.
But if you want to control the ecological footprint of Bitcoins, you should look into the USD/Difficulty ratio, that is how profitable mining is. Some miners operate on a tight margin, so getting that ratio lower would make them quit, and vice versa - get it high enough and people would want to mine with the least efficient hardware they can find.
Are you asking what impact the Bitcoin mining has on ecology and how to control it, or the economical benefits of mining rather than folding or the like, because the later part of your question is not quite clear. Could you clarify your question a bit? – ThePiachu – 2012-01-07T03:54:17.990
Sorry, you are right. The first is a practical question and the latter is just a thought in comparison. Some clustering CPU power projects are easier to justify footprint impact than others. – cyphunk – 2012-01-07T13:06:31.260