- I am assuming difficulty will rise 30% every ten days
I'm going to point out that since I started messing with Bitcoin last week, the estimated time it would take to find a block doubled. That seems less like 30% and more like 100%.
I moved from a daily average of 12 btc to a current daily reward of 8 btc in one month, due to the recent difficulty increase.
I've a an ati 5870, ~330khash/sec