Every four years, the average BTC generator will need to spend double the effort to create the same amount of coins.
As I understood it people will not need to double their CPU power to get the same amount of coins, rather there'd be half as many coins "up for grabs" by anybody with enough CPU power.
In other words, let's say a Powerful CPU can pull in 500 coins a day and a weaker CPU can pull in 300 coins per day from a pool of 10,500,000.
After 4 years, they are still pulling in 500 coins and 300 coins respectively, but now they can only access a pool of 5,250,000.
I could be wrong though. Somebody who know what they're talking about please comment instead of me!