I just had the pleasure of reading an interview with David Tennant for the 400th issue of Doctor Who Magazine. Besides being his usual charming and witty self, David gave us a tiny insight into the Doctor at the start of 2009's Christmas special.

"It doesn't carry on immediately from the end of series four. It was going to in the first draft of the script, but then it was decided that the ending of episode thirteen, was so kind of momentous and bleak that actually the Doctor may just need a bit of time out. An undisclosed amount of time has passed, and he lands in 1851, in London. It's Christmas Eve, and he's having quite a nice time until he hears...well, until it all starts to go wrong."

I am actually really pleased with this decision. I agree that he'd need some time to get over the huge losses of both Rose and Donna, as well as having to confront the Dales again. Plus, I can see him being rather gun shy at getting another companion right away, seeing as how well using Martha as his rebound companion worked out (as much as he didn't mean to hurt her).

Plus, this way they can leave the angst out of the script without offending anyone. He's had his time to mourn, we just missed it.