Now that KNC and BitFury are in full force, the diff is going to be rapidly rising. Once BFL starts delivering 600GH/s units, it's going to keep going up. By March '14, mining will be measured in TH/s, and anything less than that will barely be worth it.
I did some math on
http://mining.thegenesisblock.com/ . A VERY conservative 65% growth per month means the diff will be 2.3Billion by March. Starting in March at that diff, keeping with the 65% growth, and going for 6 months (until Sept '14), we're looking at earnings of <$100. Actually less than $90.
Why would I pay $1500 today for $90 almost a year from now?