willmoss and forever-d are onto something that most everyone has missed.

As I see it, the product of the computations is a "super-signature" on the block chain.  A block solution carries the hashing community's endorsement of the transaction history and attests to its completeness.  This is what protects receivers from double spends.  Far from being wasteful or frivolous, this resource is essential to the currency's usefulness.  Those hash values that start with all those zeroes function like a digital signature on email, but rather than a person or bank, it is the Bitcoin hashing community whose approval they demonstrate.

Bitcoin provides very little economic incentive to expend energy that would otherwise be usable.  It provides plenty of incentive to salvage energy that would go to waste if not for Bitcoin.  If someone is paying for electricity to mine bitcoins, something is wrong.  Solar panels fit the model at sunny times when nobody but Bitcoin needs their output.

The electricity spent in hashing is not wasted. It creates a product of value to the Bitcoin economy. The product is a supersignature on the complete list of transactions to date (the Block chain). This supersignature attesting to the chain's completeness is Bitcoin's defense against double spending.

Many sources of energy vary in their availability in ways that do not match the variations in demand. The law of supply and demand should require Bitcoin to soak up a lot of energy that is currently "wasted" before it makes a big dent in the otherwise usable energy supply. [Perhaps cite estimates of the break-even point for mining profitability that imply near-zero-cost electricity.]

I plan to replace the reply at https://en.bitcoin.it/wiki/Myths#Bitcoin_mining_is_a_waste_of_energy_and_harmful_for_ecology with the above text, or a revision thereof if comments so warrant...