The BBC has reiterated what I've been saying for the past year or so (in regards to fans leaving messages for David Tennant on my personal blog):

BBC STATEMENT:

There have been several recent cases of people posing as David Tennant and other members of the Doctor Who cast in Internet chat rooms and forums - as well as setting up social networking profiles in their names.

So fans are not disappointed or misled, we'd like to point out that both David Tennant and incoming Eleventh Doctor Matt Smith do not use social networking websites, chatrooms or forums. They wouldn't want fans to think that anyone who uses these are talking to the real actors.

For their own safety, we would strongly advise all Doctor Who fans to ignore any such postings and not be fooled by these impersonators

See? David Tennant really doesn't read--has never read--and will likely never read--my blog! Most certainly he never in his life will ever see any messages left for him on here...unless I post them to him myself, and considering how difficult it is for me, just to get to a blinking post office around here...that isn't very likely, sorry.