Animated capes are here

Aurora, Living Lava, Thunderstorm, Prism, Galaxy, Heartbeat and Code are now in the store.

These aren't a stretched gif: every cape has between 16 and 24 hand-drawn, high-resolution frames, played back at 20 per second and kept in sync for everyone who sees you. Cape physics are the same as vanilla Minecraft, so it moves exactly the way you'd expect when you sprint, jump or crouch.