The headline says "Billie Piper to leave Dr Who"; I think she does well on the series, so I'll be sorry to see her go. That said, I'm used to the idea that the Doctor has a fairly high turnover of companions. There's a handy list here, and it looks as if two years is about par for the course.
I don't know about the actress's reasons for leaving, but maybe she's just tired? Since Dr Who has quite a small core cast, she'll be in most of the scenes, whereas with an ensemble cast like Lost there are people who may only turn up for 2 minutes in a particular episode. Also, 26 episodes is quite long for a British TV series (e.g. Red Dwarf was 6-8 episodes per series).
What I would say, though, is that I'm a bit disappointed to see this announced in advance. That's not a dig at any particular people who've repeated it elsewhere, but it ties into my general spoiler-phobia. For instance, when Rose was dying at the end of the last series, it would have been a lot tenser if I didn't know that she was already signed up for the Christmas special and the first few episodes of the following series (ditto for the imminent regeneration). When I was a kid, those things always took me by surprise. I don't know whether that's just because I'm spending more time on the internet nowadays, or whether the same information would have appeared in tabloids back in the day ("Bonnie Langford to quit Dr Who!"), but it's a shame. It doesn't matter for something like a Bond film, but it actually ties into the story here.
I used to follow the comic newsgroups (rec.arts.comics.*), but one reason that I stopped is because they'd be full of spoilers. Nowadays, that seems to be a problem even with the comics themselves - thinking about "No Man's Land" in the Batman comics and "Civil War" in the Marvel Universe, they both had prelude storylines where a character would head off to Washington to debate the issue. And in both cases, the cover is basically telling you how this part of the storyline will end, e.g. Bruce Wayne isn't going to convince the government to keep Gotham open because if he does then it won't become a No Man's Land.
There's a similar issue with TV programs, where they like to show you what's about to happen at the start of the episode. In fairness, this isn't entirely new: The A Team and Knight Rider used to do it back in the 80s, but it didn't bother me so much then. Battlestar Galactica does it now, but they have the mitigating factor that they just show pictures with a fixed piece of music playing over the top, so all I have to do is close my eyes until it's over. In the case of Dr Who, the BBC will say "Here's a clip from the episode", and then I have to dive for the remote to mute the sound - the trick is then to hold my hand in such a position that I can tell when the clip's finished (based on the corners of the screen), but not see what's actually happening there.
I sometimes have to wonder whether I'm alone in this. I can understand the marketing approach, of trying to persuade people to watch/read something who might otherwise ignore it (although some would question whether this actually counts as news that the BBC should be reporting), but what if you've already decided that you want to watch it? Maybe I'm unusual in that I actually like to be surprised (within reason!). So, a quick poll:
If you're about to watch a program, do you want to see highlights first?
Yes, tell me now, I don't want to wait!
No, I want to find out during the story.
Ending on a more positive note, I wouldn't discount the possibility of Rose and/or the 9th Doctor showing up again - now that they've done a regeneration in the "new" series, and brought in a former assistant, they've laid the groundwork for a "The N doctors" story.