I can confirm that the bug still exists.
I did the above slightly differently, while the horse is in the ground, mount it and then jump 🙂
It has been patched, although horses still fall into the world if you break the mine cart their in.
Oh yeah lol.
Also, here's a recording of a creative glitch: http://youtu.be/CIKk1K3I6Bo
May still be uploading.
http://www.youtube.com/watch?v=CIKk1K3I6Bo
Here's a recording, it's uploading, will be done in about 6 mins.
Not really Marcono1234, it just involves some relatively simple changes to the way Minecarts work.