Once you upload a skin to minecraft.net the old one is deleted from there, it holds one per user. you will have to upload the old skin again to get it back on your player.
http://minecraft-skin-viewer.com/ claims to have both old and current skins, I don't know, my change doesn't seem to show up on any of these sites even though I can see my new look in game they all show the Steve skin for me.
So basically, I'm stupid.
Just testing.