They get tougher ever 14 days, relative only to the processing power of the network.
It keeps the currency from farmed rapidly and devaluing.
Simply, the more power there is to crack them, the harder it is to crack them, and the overall generation of bitcoins is kept on a constant.
If you have a decent ATI Mult-GPU rig capable of 800+mh/s I'd just join a mining pool and get small constant payouts..
Still trying to do the numbers on getting some asics and riding the first or two upticks in difficulty adjustment vs payout.