There was a news article just the other day saying it was all but a done deal and of course I can't find the article now, it definately said that it would be a series by the BBC, rather than tv movie or mini-series the one thing it didnt specify was just how much involvement Joss Whedon is to have.
Edit: had another look and although I didnt find the arcticle i was after it looks like what will be confirmed soon is that it will enter development, so well short of a definite to hit our screens but still its getting more likely rather than less
.