From Doctor Who Magazine issue 391, in a Q&A session with the Doctor himself, someone asked him about his favourite TV series; he mentioned Torchwood, and the Sarah Jane Adventures (obvously) but also:
"Actually I Watched Firefly recently, and that was terrific, but it was topped by Serenity which is brilliant. He's very clever, Joss Whedon"
Later on when talking about his tapdancing skills, he also remarks in passing "...I think Buffy nailed the musical episode..."
Who else wants Joss to write/direct an episode of Doctor Who? or Torchwood - after all James Masters is in the first episode of Torchwood this week.
sorry, slightly off-topic.
A Thought:Tennant could be a browncoat, he's worn one all last season (though it's quite amusing to think what Mal and Zoe would make of the type of coat...