I think it is caused by fatigue because the spoiler moves about a bit. I've side swiped the kerb a few times (a bit too sideways!) and this lateral loading have probably caused the cracks on my spoiler, plus the fact that I have done lots of trackday and I run a very hard suspension setup on the road.
It would be interesting to know if anyone else has this problem, it's the sort of thing you wouldn't notice unless you are looking for it. It shouldn't happen so get it sorted on warranty.
Thanks Clive, I've not had mine on a track and rarely go sideways, my suspension is standard. It's very noticable because my car is black and the plastic(?) beneath is white. I've taken it back to the dealer and they are talking to Colt.