There are no rules I can find for it. Teleport says nothing about creating any condition (like the target is unaware of your position even for a split second), no rules for stealth saying you can do it after a teleport or similar effect, and no rules under combat advantage or flanking (such as flanking with yourself after a teleport - yes, I was grasping at straws by that point).
My gut says this: teleporting is super cool, but using it to give yourself combat advantage is a direct line to some nasty combos that as a DM I might regret. There are some rogue powers that, once you have combat advantage, make it hard for you to lose it. You can for sure use it to teleport into a flanking position to gain combat advantage, which is pretty nice.
I'd like to allow that, but I personally won't be allowing it until I see how often combat advantage is available and how much of an impact it has on the game. I haven't played enough 4e yet to know. Plus, doing it this way means my players might get to feel like they've gained something, instead of me realizing it's too powerful for my game and having the players now feel like they've lost something. I can't imagine it would really BREAK anything, but sometimes it's all about perception in terms of gaining and losing abilities.