Cant stress this enough ... there is not a SINGLE rule or ability in all of 4E that allows you to take an OA if someone shifts, teleports or force moves.
Actually, I can allow one of my allies to do this every single round of my career, starting at first level, if I'm a Warlord.
Check out the at-will power Viper's Strike. I guess the sanctity of "No Opportunity Attacks" when shifting is not so inviolate.
I was going to make all of my points and arguments on this issue, but Hypersmurf already stated everything that I was going to. So I agree with him.
Shifting provokes Polearm Gamble, but teleporting and forced movement do not. The rules seem really clear to me on the matter.
Also, just consider the spirit of the rules here.
Shifting is meant to be a defensive form of movement, a means of carefully extracting oneself from harm's way, or maneuvering cautiously around an opponent, circling at arm's length so to speak. It's not intended primarily as a way to approach an enemy, to engage. No one ever talks about "shifting" up to a foe to attack, and in normal circumstances, you don't need to, as moving INTO adjacency with an enemy doesn't provoke. Just moving around or away when you're already adjacent does. I think that this is precisely why the distinction is drawn in the rules language between teleporting/forced movement being immune to opportunity attacks in an unqualified respect, and shifting being explicitly described as applying to movement out of an adjacent square.
Polearm Gamble is supposed to be you using your superior reach to prevent someone from closing on you, or at least to punish them badly for doing so. It's saying, "Hey, I'm not letting you get close, I have you out at the end of this polearm, and if you take one more step, I'm sticking you before you can get into range to attack me." There's no reason why shifting (which makes tons of sense considered as a careful withdrawal or guarded sidestepping) would somehow bypass this. The polearm is still pointed at you, tensed and ready to thrust as soon as you move closer. Moving more slowly isn't going to stop that. I'm not going to stand there and just watch you sidle up to me without striking with my clear reach advantage, just because you didn't move forward as quickly as normal.
Consider, also, that the REASON you're getting this opportunity attack is different from a normal situation. Normally, you're getting to take an extra slice at someone because they dropped their guard, they tried to get away or get around you without covering their movement, so you have a special opportunity to capitalize on their momentary lack of wariness. But with Polearm Gamble, the opportunity attack has nothing to do with that. It's giving you the chance to make a free attack in a situation which normally never grants one: an enemy closing to melee with you. It's obviously assumed that enemies moving INTO your adjacent zone are not dropping their guards, or else everyone would get opportunity attacks any time a foe approached. So in this case, the opportunity attack is granted solely on the fact that you have this long reach weapon, and you're specially trained to wield it to your advantage in these cases (as evidenced by the fact that you've taken this paragon-level feat to do so). So it doesn't matter whether the enemy is guarding himself or not, it only matters that you've got a polearm between yourself and him, and he's NOT getting past it without you having a chance to strike first. He can "shift" up as slowly and carefully as he wants, if he tries to just waltz past your readied polearm, you're going to take a shot at him.
I think the rules can potentially be interpreted at least a couple of different ways, given a strict reading. I can admit that the other interpretation cannot be wholly discounted with logical arguments. But I think that considering the spirit of the rules, and of the feat, and what seems to me to be the designers' intent, the interpretation which defines shift's "no opportunity attacks" property as being specific to moving out of an adjacent square is more fitting. It just makes sense for Polearm Gamble to ignore shifting, and the rules lawyering which supports that view is at least equally valid and compelling as that which would refute it.
That's how I decide on rules questions. After I read every available scrap of official rules text, if there are still two or more completely sound ways of interpreting the words as written, I try to divine the spirit and intent of the game, and go with the rules judgment which most closely aligns with that.
Anyone who wants to run it the other way in your own games, feel free. But I think that you're doing a disservice to that feat, and to characters built around a polearm concept (which there is already so little support for in the game thus far), by allowing enemies to so easily bypass what is meant to be a fairly formidable defense/control tactic, with something as simple and ubiquitous as a lowly shift.