Then there is the problem Clarke discusses in 3001, abundant energy of this sort will result in more warming, not less. I found strawberries on monday 4 lbs for $4. I bought 12 lbs of strawberries. At $3.99/lb, I find myself reluctant by 1 lb. I can't say that what I did was uneconomical though, 8 lbs went into 20 jars of jam.
I love strawberry jam, but I won't jam at $3.99/lb, when I can pick blackberries in my backyard.
If power becomes too cheap to meter (which would be the likely outcome of such a device), then there is never any worry about leaving lightbulbs burning, saving gas, etc.
just for interest, steorn have a fun section.
note the many young and female members of their forum; they have obviously reversed the rule that 90% of engineers are male as well !