http://blockchain.info/charts/miners-operating-profit-margin
Mining ROI chart, as compared to the network hash chart-
http://blockchain.info/charts/hash-rate
These are inverse, meaning of course mining ROI is decreasing with the rise in network power, currently rocking a solid 15% margin or so. Looking at the market valuation-
http://blockchain.info/charts/market-price
We can see the value of Bitcoin has maintained a more or less flat value (very erratic to say the least really, I know, but bare with me), but holding between $90-120.
We must also factor in that as designed, only less and less coins can be shaken from a block over time as well.
Analyzing this you can see the margin chart was more or less parallel to the market prices, then around the beginning of June mining ROI started a nosedive from around 50% to about 20%, a pretty fatal drop in returns in a very short period. During the same period the hashrate started out at around 76,000 GHash, and during that 30% ROI slide the hashrate rose to around 150,000 Ghash for nearly a 50% increase.
These numbers are only increasing currently.
So, each Bitcoin mined is only harder and harder to get, but is not increasing in value fast enough to offset the hashpower rocketship to make mining worth it. If BTC value was increasing alongside the hashpower increase the mining ROI chart wouldn't be diving the way it is currently, either maintaining the return rate or increasing.
What will happen is the markets will basically decide if this dramatic rise in network power is worth it to maintain. If Bitcoin maintains its current value then this "mining bubble" will pop when ROI effectively vanishes completely, and some miners will have a pretty bad day holding the bag on expensive hardware that will take years to pay itself off. I think we are inching close to a plateau if Bitcoin fails to increase in value itself soon. A plateau in hashpower would indicate no new power is coming online, which means Avalon and the rest of the ASIC hopefuls will see their sales flatline for a while.
Be warned, we are nearing a saturation point where it won't be worth it to buy a Bitcoin miner without a dramatic increase in Bitcoin's market price to sustain profitability. Really those that were able to get in with the first Avalon's and ASICMiner blades already won the race, anyone getting in now faces a ugly situation.
What his means is miners need to come out of their caves and get local businesses and people actually using this coin as intended to raise value and market price to stay alive before the eventual bust.
Now, I have no crystal ball, this is just the current situation and outlook based on preliminary information. Your ROI is also depending on your power and support costs to mine, so this is all in a general sense. Bitcoin could have a massive rally tomorrow for all I know (and I hope it does every day
) and save the day. But counting on that to save you is reckless, and is just the risks of the mining game. Just think about this before plunging any money into an ASIC right now, and place your bets wisely.