Here's a post Markus Persson made on Reddit, back in the days, in 2011.
But now, as apparently we can edit a cape to our character without the need for a mod, capes are less unique and "premium".... Unless special capes give special effects.
... I can't really test that out, since I recently decided to migrate my old Minecraft.net account to the Mojang system.... And I have major issues relating to that.