There's an easy way to tell where the difficulty rate should end up depending on the current price of bitcoin needed to cover electricity costs.
The "natural" position for the difficulty to come to a rest at, as we know, is at a place where the proceeds from mining is just over the cost of electricity needed to mine.
So, look at the difficulty per month that corresponds to the current price of bitcoin and that's the upper limit of difficulty, realistically.
So here I have a readout. 53 BTC is the cost of one Jupiter paid the first week when Bitcoin was $130, and hosting was another 22 BTC--75 total, but you have to remember you're not paying for electricity costs anymore so set that to zero, and 520 gh/s is the 30% improvement over 400 they mentioned (and that's probably still low).
A Jupiter, depending on local power costs, costs between $50 and $100 a month to run. So if the price of bitcoin is $125 as it is right now, then we can mine profitably until ~1 btc a month is generated.
Which on that image is between April and May of next year, assuming it's close to right (which is questionable).
But notice the difficulty on the left that corresponds to that mining amount: 7 billion difficulty.
7 billion is the upper limit of difficulty at a $125 price.
Now there's several good reasons why the difficulty will -not- get to 7 billion with the price of bitcoin at $125. There's ROI to think about, the cost of hardware must be amortized in, which means the difficulty is actually very likely to level off at more around 2 billion or less.
At 1 billion we're making 6 btc a month, at 2 billion it's 3 per month. Not bad. We will have long since achieved positive ROI.
If the price of bitcoin remains at less than $200 next year, we will be cemented in place as the new mining kings. Because, having run the difficulty up to its natural place, there's not enough low-hanging rapid profit left for people to invest in miners. Too risky, not enough quick profit.
We're going to ROI in a month and change. Anyone buying a miner in January might have to wait a year to ROI, if they ever can achieve it at all.
Now, all bets are off if bitcoin runs up to $1,000 next year, and then everyone buying miners now will look like a genius, and here's the scary and hard to even think about part--we won't even mind making only a few tenths of a bitcoin per month at that point, because that will be a couple hundred dollars.
And the mining game will start all over again
March 2014, here we come.