you can get 2 x 200 GH/s machines for 5K
they will mine 1BTC a week between them at current difficulty.
with increased difficulty you sould get ROI in 7 weeks.
Seems pretty good to me.
How accurate are profitability calculators? According to all of the ones I've checked, profitability will go way down in about 5 months from now. It seems that the next three to four months are all that are left for mining in the black.
I want to jump in, but difficulty is not making an investment in hardware very attractive.
It's a risk you have to take. Do you believe that the 3 TH machines will be released on schedule, and are you slated to get one? Alternatively, do you have access to the latest 650 GH/s machines?
If you're not, I wouldn't bother mining BTC. - Look into alt-coins or arbitrage in the financial markets (ie, trade 1 crypto currency for another)
My personal estimate is that a 650 GH/s machine has about 14 BTC left in its life, so if you want to get one, bid appropriately.
disclaimer - I have a batch 1 KNC Neptune on order that I'm thinking about cancelling since the batch 2 comes out a month later, and if it follows KNC's historical info, it will probably be faster