We've had flash-based pony creations tools pop up before, and they did a pretty good job of emulating the show's art style while still fulfilling that need to see your personal ponies rendered in pleasing pixelated... I don't know, I was just using a lot of P words.
For those who are interested in that sort of thing, we now have access to a 3D pony creator that was created by the Ponykart team lead, Hoppip. It's very easy to use, though I had some problems with it accepting my cutie mark via the image picker. I had to override the cutie mark that loaded by default to get it to show. It's probably a problem on my end, as no one else had this issue. I think filly Cereal!pony came out well anyway!
You can grab the creator here.