HD skins will never be implemented. Jeb says it may cause lag for some people, and if you have a griefer with, let's say, 1000x1000 skin, it will crash everybody's client on the server.
A solution for single player is a texture pack. Alter the char.png in the mobs folder.
I am not sure if this is what you are talking about or not, you might just mean for a texture pack :/ Either way, I hope this helps someone out there.
I know this thread is years old but I may have a solution to this "HD skins thing" Why not just have a client option to only view the standard resolution of the skin and those players with more beefy computers can view the HD skin.