Recently, at the San Diego Comic-Con, Matt Smith was interviewed by IGN.
In the interview, Smith refers to the Christmas Special as "the final hour" and continues by saying that Moffat's script is "20 pages in" but he hasn't read it through yet.
When he was asked about whether he knew anything about who the next Doctor is he said "As far as I’m aware they’re looking, or they’re beginning to look and they’ve got quite a while until they need to decide. So I imagine it’ll be someone fresh to be honest, but I really have no idea."
Smith is also asked about who he believes the next Doctor should be and all he said was "Whoever it is will be will be the right actor…or actress. Let’s wait and see… who she is."
The full interview is below:
[Source: Doctor Who TV]