In reply to philhilo:
Increasing weight on problems/routes you can already do will of course make them harder and up the intensity which is the essence of progressive training.
However most people just up the difficulty which is exactly what you're aim is on the rock. Doing harder routes will make you more able to see what you can and can't do. It'll teach you do harder moves. Part of progressing is knowing that you're capable of pulling on certain sized holds in certain positions. Also adding weights will alter you're centre of balance which you might have to relearn when you take the weights off.
In short it'll help but probably not the best way. However just to confuse things even more I have a friend who boulders with a weight belt. He's very strong and reckons its great way to train.