The bitcoin mining difficulty retargets about every two weeks, so that the rate of transaction blocks still is about six an hour when network hashrate changes. For the past two difficulty recalculations, there has been a slight
decrease in difficulty.
Bitcoin block-finding is a random event; even on the largest pools you don't make a constant daily amount when they use a payment system that rewards miners only when a block is found. On a small pool, you might be rewarded for two block finds in a day, then wait several days for the next. The size of the pool or the interval between block finds doesn't affect your expected rewards though - by pooling your mining resources and splitting the rewards with others, you just get more regular payments than the one-in-many-months reward that would come with solo mining.
This post of mine has historical difficulty data.
