If you try and mine with a cpu it should point and laugh or say "youre doing it wrong".
WAAAAAY back in May of last year I did a little CPU mining. Then stopped when I realized I was spending more on electricity than the bitcoins I generated were worth. Bitcoins were selling for under a penny a piece, and I figured it cost a couple cents in electricity to mine them.
So if you think there's a chance bitcoin prices will be 10 or 100 times higher in the next year or three, maybe CPU mining now isn't crazy.
Just saying.
From an expected value perspective, this could potentially make cpu mining worthwhile. But from a utility perspective, it doesn't matter the overall expected value if your chances of actually collecting anything are minute or non-existent. Just like people will not pay an arbitrary fee to join the St. Petersburg lottery, nobody will CPU mine if the average time to payout is 7 years.