CPU mining is ludicrous, it would take you a year mining 24/7 on a new i5 CPU to even make enough Bitcoin to meet the minimum withdraw amount on pools, while costing ten times more in electricity than your earnings.
Anyone else care to take a stab at this explanation? I don't think I did it justice.
http://we.lovebitco.in/mining/So you are saying that 8 i5s could do the same job in 1.5 months. Granted. i5s us a lot of power, probably more than you could make in coins. But Atom processors are relatively cheap and low power. Granted they are slower as well.