I have a son the same age. He tried lying a year or two ago and got caught in it and we took all electronics away for one week for each time he told the lie to us before he finally gave in and told the truth, came up to be 6 weeks. That was no Wii, TV, computer, DS, anything, all he could do was read and do homework. He got the message and doesn't lie anymore...that or he has gotten to be damn good at it.
If it was at the point of what you are mentioning now, the xbox would disappear and I wouldn't even give him the closure of knowing what happened to it, it would just be gone, and in it's place I might leave a note that says next time you are caught lying or taking something that isn't yours it will be worse.