Gee, it gets harder and harder to make bitcoins? No shit sherlock.
And more people are learning that they can use bitcoins for anonymous online transactions? Think that makes demand go up?
It does not matter that the production pace of bitcoins goes down in nominal terms if the demand for bitcoins also goes up. The dollars per day that one would earn is still dictated by supply and demand of bitcoins relative to your ability to create bitcoins relative to your cost to produce them.
K, so someone has a pair of 6990s making 1300mash.
Lets say someone has $1 million and that computer running 6990s is $1200.
That pays for the hardware for 833 computers, making 1082900 mhash.
http://www.alloscomp.com/bitcoin/calculator.php
Plug in $20 per bitcoin... pretty common price lately. 1082900 mhash.
Today, that comes to $38,395.32 per day. $279,965.85 per week. $1,167,217.63 per month. Month is more questionable. What will they cost in a month? Not sure. I am sure that you will have a giant fucking electric bill, but probably not $1 million worth.
Now, lets see what kind of production is going on in the whole network?
http://bitcoin.sipa.be/speed-lin.png
At current prices, $1 million of hardware gives you 1/7th of the total production.
So, yes, someone with $10-20 million
CAN dominate the production of future bitcoins.
They would need to come up with a setup that uses as much passive cooling as possible to keep their electric bill low and ideally be in a place with low electric rates. That would be their competitive advantage that allows them to produce at a lower cost than is economical for their competition.