I agree also. Spirit was considered a flop by Dreamworks (you said Disney, just pointing you to the real culprit, Lol! ) since it just about scratched 90 million at the box office - I can't remember if that's US Dollars or Sterling Pounds, probably the former. That's nothing the film industry. The soundtrack probably made more than it.
It was released in 2002. At that time Spider Man came out. There's the problem. It's the same story as Balto: two brilliant films, but one of them (Toy Story and Spider Man) had a much wider audience and interest. Also, it's 9 years (wow!) since it came out, so there's little chance that they're going to revive it. Dreamworks is too occupied with Shrek (there could well be a 5th one, despite the name of the last film) and Madagascar (3rd film expected soon), and new stuff like How to Train Your Dragon. It's about money I'm afraid. Spirit failed first time round, second time would be disasterous where money is concerned.
But saying that, Monsters Inc. came out in 2001 and a sequel is in the making (apparently). I heard late 2011, but I haven't had anything since, so it might be 2012. But Monsters Inc. was a MASSIVE success. And Disney's loaded, so if they screw up on a film it won't make much difference, whereas Dreamworks are really fighting to compete with them these days.
So, no, there isn't going to be a sequel, otherwise Animation Source would have pounced on it by now. I think Spirit is perfect the way it is, and even now I feel uplifted after watching the whole movie for the millionth time.

i agree with Kirada

So far there hasn't been any official or recent news about a Spirit 2.

Personally, I think it would ruin a good movie. Perhaps following more of Esperanza and Spirit'sfather would be nice, because he (Spirit's father) wasn't even mentioned in the movie, but was in the book. As far as a continuation of Spirit and Rain, I wouldn't think so... good movies are ruined by trying too hard.