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/eta
Difficulty of next reset: 513600, based on http://blockexplorer.com/q/estimate
Days 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?