So when was it worth it? I see it's meant to run out with current trends in 2140 after 21 million coins are mined. That's still a lot of coins to be found.
But the coins are issued reverse exponentially, so the first year will release a huge portion of the coins and the last year hardly any, the people mining at last 1% will take millions of times more computation power to mine than the first 1%.
it's still worth it, and always will be. you're right - "That's still a lot of coins to be found."
Not true at all, bitcoins could go to $0 today, it then wouldn't be worth it. More likely they will increase slightly and the exponentially increasing difficulty (its actually increasing faster than exponentially right now!) will merge with price and raw power cost to mine them in about 45 days, at which point it won't even be cost effective to mine on a ATI GPU if you pay for your own power and all your hardware is already paid for.
Tomorrow we will be at the tipping point where buying a dedicated rig is no longer a return but a loss, 45 days from then even people who already own their hardware won't make money if they pay for power. This is assuming big increases in bitcoins value of $25-$50. If it increases like others hope to $100 - $1000 then you will still make money.
Like all commodity rushes the first that got in make the most while the rest fight for 5% of nothing and most lose money. The difference with bitcoin is its virtual, so even the people holding massive amounts of bitcoins can be burned in an instant.
Misuse of the term exponential aside, I'm not sure where you are getting your numbers from. I have a limited data set to work from but the previous difficulty changes have been: 79000 -> 101500 -> 157000 -> 244000 -> 435000 (estimated). Somewhere in there has been a huge flood of media attention, a large computer enthusiast forum hopping on, etc. and it's held to about 50-60% increase for difficulty/hashrate. Even assuming that this trend continues at 60% increase in difficulty every 10 days in 40 days it will yield a difficulty of about 2850816, and a hash rate of 23THash/sec (which I'd find a surprise). Even a meager 1 GPU rig of 400MHash/sec will still generate an estimated $25/wk @ $25/coin, far far far far above the cost of electricity. Even at the next 60% difficulty increase to 4561305 this same machine would generate roughly $16/wk.
Now if rates stayed stagnant @ $8.5/coin while difficulty increased > 10fold then yes, you have a point that existing mining machinery would no longer be profitable, but if that were the case I can hardly imagine such a continued pressure to add computing power to the pool.
I'm not saying that someone is still ok buying a dedicated rig, it's become much more of a PITA and profits are hard to be found, just saying that you are making bold statements that I'm not sure the source of.
For those wondering about whether or not to build a mining rig at this moment, I'd advise extreme caution. All number crunching aside there is currently a large collection of people with $ signs in their eyes overbuying that you will be competing with entering the market, while we don't necessarily see the same headlong rush of investors into the bitcoin market. People with a real need to sell (to recoup costs) without an equivalent need to buy doesn't forecast well for offsetting the diminished returns that are already in the pipeline. This is why you will see so many people promoting the investment in bitcoins rather than mining hardware, both because it will be more profitable for them personally, as well as that it is the only way to keep the system running.