The limit on difficulty is a combination of hardware efficiency and BTC value. At $600 BTC, 28nm tech has a very hard time reaching ROI when the difficulty goes over 100B. 65 and 40 nm tech limits out much lower, so regardless of how much hashpower ships the 20% increases every 11 days are only going to continue for 3-6 more months, or until BTC value jumps again.
I personally think we will see 20 to 25B by mid-July, then BTC will go on another run.
https://bitcoinwisdom.com/bitcoin/difficultyEstimated Next Difficulty: 10,457,675,910 (+18.12%)
Adjust time: ...about 6.8 days
Looking like a pretty good prediction as it looks like it might be 14B to 18B by mid July.
If the price of BTC does not rise before the time 25B arrives, any home miner not running at 0.5 watts per GH will be worthless - income wise.
At 40B without a price shift in BTC, a 1TH/s 28 nm miner will mine 0.01257 BTC a day ( $5.52 USD ). After cooling, storage and power costs, this will leave about $0.80c
For a home miner this would be ridiculous, but for hosted mining this would represent a 15% profit margin. Enough at least to maintain, and slowly increment rather than exponential incrementation that we are seeing now - as many businesses can and do function at a 15% profit margin.
As for the price of BTC, my guess is the primary trading of this cryptocoin at the moment is done by the mining communities desparate to recover their capital purchase costs, and manufacturers dumping their earned BTC on the market. Until we see the day where I can go into town and buy some BTC the same way I can buy cell phone credit, i.e. have a shop worker hand me a till receipt / paper wallet with $20 BTC credit on it, and until the day an E-Bay that is BTC only becomes as big as EBay itself becomes established, and/or someone creates a prepaid cellphone that only takes BTC as credit, then don't expect the price of BTC to go anywhere but slowly go down.
For those that have invested their life savings into their home mining operations, this is a lesson in the nature of the beast, small independent business venture getting captured by large bank funded cronies, then having the swamp drained by them - capitalism at its finest.