The video accompanying this post doesn't include Rainbow Dash, but it does include Rarity. And, really, that's all that matters, because Rarity is better.
A new kart-racing game has popped up; an amalgamation of two separate projects merged into one, teams and all. The team is currently making good progress on the game, but they're still looking for team members, including 3D modellers and concept artists. You can find the Ponychan collab thread here, or you can also email the project lead directly here.
Click after the break to see a proof of concept video. The email clarifies that the final version most likely will have the ponies in karts and not feature Mariokart tracks.