I dont think Julian is refering to grout 'dust' left on the tiles, more like a definite 'layer' of waterproof grout left on them. If that is the case, then you can 'buff' with a microfibre cloth till the cows come home!
We complained to a Bovis site manager recently, about house after house with this very problem, the tilers had to come back and 'finish' the job!
If it is a 'one off', then maybe, grit your teeth and get on with it.