Steven Moffat -- the incredible mind behind the best episodes of Doctor Who such as "Blink", "The Empty Child", and "The Doctor Dances" -- will be taking over as lead writer and executive producer for Series 5!
This is fantastic news. Moffat is brilliant. Russel T Davies, the current Exec, has done some great things with Doctor Who, and was largely responsible for getting it back on the air. However, I think he's running out of steam -- so far this season only a couple of episodes have really stood out, and the rest have been a bit plodding and silly -- and new blood will help tremendously.
I can't think of anyone better than Moffat to take over (and it's been widely rumored for months that he would). If you've ever watched the UK show "Coupling" you know what I mean. Moffat is extremely clever, and as he gets familiar with a show he takes more risks rather than let things coast. He shows scenes out of order for comedic effect, or teases an idea throughout an episode, or just takes a simple idea and runs it through its complete course, taking no prisoners.
So: yay! The only bad news: Series 5 won't start until 2010, after they take a year off. That's a bummer, but I think it'll be worth the wait.
Tip o' the wibbly-wobbly timey-wimey stuff to Gia. Mwah!