I have been experiencing this problem for a while, sometimes Minecraft will simply start me over on a world. Same world and all, but I go back to spawn, all the stuff in my inventory disappears, and my beautiful skin changes to Steve or Alex. It usually happens on LAN games, but I have also had it on single player worlds, and my bro looked at the .minecraft folder and apparently it just creates a whole new player file, with the old one still in there.
What really gets me is I took a challenge to try to survive in Alpha and it did the same thing. If this problem dates back to Alpha, why has nobody fixed it yet?
If anyone has a solution, plz tell me, otherwise someone tell Mojang to get to work on this.