Matt Smith has told the ITV show Daybreak at the GQ Awards that he will be begin filming his last story which is this years Christmas Special and also explains what they are going to do with his hair. Here is the quote.
I did it for a film, but it means I have to wear a wig in Doctor Who which is a nightmare. I'm excited, I start shooting the Christmas Special Sunday. It'll be sad - the end of a wonderful era!My idea for the hair would be to have the Cybermen shave it off while trying to convert him into a Cyberman. But anyway I guess it will be a wig then.