Here's another way of looking at the potential for continued long-term exponential growth (but I think it uses the same line of thinking)...
+1. This is what I meant. Thanks for the down-to-earth calculation. Don't let TheGenesisBlock's calculator (which is awesome, btw) trick you into thinking we're going to have a larger (hashrate-wise) network than we will. Remember - nearly everyone advising you against mining .. is a miner 
. I would still take delivery on a $1299 60GH miner.