That would be my instinct as well.
As for balance, the only abusive issue I've seen with this is the Arcane Wayfarer PP, which gets a speed teleport 2 at 16th level. Combining all these effects give an eladrin a huge teleport speed usable constantly.
See:
Warlock U10: Ethereal Sidestep. (then you go slasher at paragon, of course). At will teleport 1.
That said, instinct or no, similar bonuses -do- stack. Only bonuses that are one of the following don't stack:
1. Typed bonuses of the same type boosting the same thing (e.g. a feat bonus to speed, another feat bonus to speed).
2. Untyped bonuses From the same-named game element (eg, the same bonus from the same element).
Multiple bonuses to the same thing from -different- sources always stack -- like, say, having a ring that boosts your teleport speed and some boots that also boost your teleport speed. Frankly, this isn't a big deal; this is what these things are supposed to do, and having the ability to teleport yourself very large distance is exactly the opposite of game breaking.
It's only when you can teleport -enemies- an arbitrary distance that this can get abusive (as against non-flying enemies and outside, an arbitrarily large teleport is a save or die). So the items to watch are Gloves of Dimensional Repulsion (+2 to hostile teleports), Ring of Retreat (+1 to all teleport powers), and ring of calling (give it to your allies, then stand adjacent to them for a +4 to teleport powers). Even there, you're adding an extra 3.5d10 to hostile teleport powers for 3 paragon items (so hostile teleport dpr += 5.5*.25 (hit chance * save failing chance, keeping in mind that elites and solos make this worse) * 3.5 = 4.8 -- which is nifty if you're making hostile teleports as your main attack form, but not game breaking for three stacked items in decent slots.
Of course, the truly ugly thing you can do with teleports has nothing to do with increasing how big your teleports are (though getting at will teleports up to at least size 2 for that build is a must), and everything to do with making sure you teleport as many times a turn as possible combined with a slasher feature (auto damage adjacent foes on a teleport). Oh, and adding damage types to that feature and making foes vulnerable to them, of course (Long Night Scion has a leg up here, as they start with an excellent damage type on their slashing already). Planestrider boots to make every teleport 2 teleports, at will teleports from Etherial Sidestep; a minor action teleport if you can get one (hmm. aside from shifty, what about a druid multiclass? Wield a staff of the traveller, Agile Form, and wear something that boosts teleport size (Eladrin ring of passage or whatever) and you're making a minor action teleport 2 (split into two teleports) every turn, in addition to the teleports you get from your attacks (all of which shift or teleport you) and your move action (always a teleport). Oh, and for a feat, that becomes a free action 1/turn slash, and you can find something else to do with your minor actions. don't think I've seen a druid slasher build, actually; might be a fun exercise).
