In reply to Fellover:
Most fork manufacturers, and some wheel manufacturers will give a maximum disc brake size; its there for warranty purposes, but unless you weigh a LOT and are really pushing things to the limit, you will be fine. There will always be some people who've 'heard' of horror stories, but in reality, the number of people who've actually snapped a decent fork or ripped their spokes due to a too-large rotor must be countable on one hand. Obviously, perhaps avoid trying to stick a 203mm on a 140mm though! Basically; I wouldn't worry.
My xc mtb is posted for 160mm all round and stated that 180/160mm is the max. However, I've been riding 203mm/180 for a year now with no problems (other than having once gone over the handlebars into a tree, because I grabbed a handful soon after fitting them).