The switch to 25 BTC will happen at block 210,000, which will happen in January 2012. https://en.bitcoin.it/wiki/File:Total_bitcoins_over_time_gra... Just like how GPU mining made CPU mining unprofitable, something else could come along and make GPU mining unprofitable. Probably couldn't happen before you've gotten your hardware purchase paid off though. Of course, if the exchange rate drops significantly, like the scenario pointed out here: ( http://mtgox.com/blog/?p=262 ) and doesn't recover for a while, you could end up never covering the cost of the equipment for your rig.