I put together my own Google spreadsheet to examine whether it makes sense to add to what I am currently using to mine, and the numbers keep looking bad. E.g. for a triple 5830 system:
Days to next reset:5.0, based on
http://blockexplorer.com/q/etaDifficulty of next reset: 513600, based on
http://blockexplorer.com/q/estimateDays per difficulty increase after the next one: 8
Difficulty increase factor after the next one: 1.3 (30% average increase)
Watts: 550
$/KWh: 0.12
MHash/Sec: 840 (3 * 280 per 5830)
$/Bitcoin: 10
Running the above numbers, on August 18 the electric cost exceeds the generated bitcoin value. Total income at that point (assuming selling bitcoins at $10 apiece) is $492 - putting together a 3-way 5830 system for $492 is pretty tough (used parts maybe?)
Are other people using more optimistic numbers?
How about this, at no time in the last year has bitcoin cost more than 1/4 of its exchange value to generate based on electricity. It's currently at about 1/17th the value to generate (this fluctuates over time and just left an all time high). People have been doing calculations like you have for months saying the same thing you have