ChrisS - I wouldn't worry about it too much. I discovered what appeared to be a crack in one of my Zaskar Carbons.
I rang a company called Carbon Frame Repairs (01530 451664) and they told me that, 99% of the time, "cracks" appear because the carbon flexes (naturally), and the laquer/paint does not. This is especially true with frames that have aluminium lugs at the end of the carbon tubes, and people will quite often notice a hairline, dead straight crack in the paint, and think it's a crack in the frame itself. The guy I spoke to couldn't understand why bike companies spray/lacquer over an area of the frame that is more subject to stress/flex (as in my case), as it just leads to an increase in warrantly claims when, in fact, there's absolutely nothing wrong with the frame.
I did exactly what monkeyp above has suggested (and what the guy at the carbon frame repairers said), and I marked each end of the crack with a marker. It hasn't spread. In my case the "crack" was in a relatively high-stress area (where the seatstay meets the dropout) and it hasn't budged. I'm 100% confident that it's the paint and not the frame.