Thought id should post a reply to this thread seeing as i had the same problem over the weekend. Not really sure how it happend, could be to do with preasure on the screen while being in my pocket etc, but not 100% sure. But i took the phone back to the vodafone store, i did originally buy it online, but after typing out a email to vodafone via the website, they suggested to take it to my nearest store if possible, so i did.
The phone is only 3 weeks old to me, and the gentleman in the shop replaced it there and then. At first when i showed him the phone from a distance he thought it had contact with water, but after a close examination he said "weird, not sure what that is" and replaced it on the spot. Really annoying to be honest because now im abit cautious with putting the phone in my pocket! If it happens again i will probably try and get a full refund and put some extra cash towards a new phone. I dont really wana do that becaues this phone is just to good for the money!