Hm, pretty ambiguous: p285 of the PHB says forced movement "does not provoke opportunity attacks or other opportunity actions," but nothing about other events triggered by movement. Also:
Not a Move: Forced movement doesn’t count
against a target’s ability to move on its turn. A target’s
speed is irrelevant to the distance you move it.
That title is pretty blantant that forced movement is not a move - except that the text is much narrower.
I'd have said that forced move doesn't trigger the damage from this, but it's obviously arguable, and if it's the same language as Wall of Fire and the FAQ says Wall of Fire should be triggered by forced move, I guess I have no objection to that.
I would regard putting it above someone as being slightly tricky with the rules, and I'd give them a situational way out - for instance, I'd let them avoid damage by crawling beneath it, even though by RAW it should fill the entire square below it. I'd leave that up to the DM.
Two things that I note about AP:
There are a bunch of Illusion powers reprinted from Dragon, which we've already approved, but there have been a few wording changes. IIRC the only one that's significant is Illusory Wall - the old version made no senes, and the new version makes no sense in a different way. I'll need to dig out my old post about that. Anyway, we'll need to decide whether we're treating this as errata to the Dragon powers (disallow that article when we allow AP) or what.
The Bonded Summoner paragon path has a similar question as Storm Pillar: the Planar Gateway power creates a conjuration that has a "free action" (not an opportunity action) with a trigger, that when a character "starts its turn in the planar gateway or enters a square adjacent to it", the caster does damage and can shift it 3 squares. If forced movement counts as "entering" a square, then this lets the caster slide the target back and forth between adjacent squares forever, taking damage each time.
Clearly this should not be possible, but we need to decide how to fix it: clarify that forced movement doesn't count as "entering", so forcing someone next to the gateway never has an effect, or adding a clause saying that forced movement as part of the power doesn't trigger the power again, so you can throw somebody into the gateway once (I prefer this, as throwing somebody into the gateway is nice and cinematic, especially if we're allowing forced movement for Storm Pillar - the only difference in wording is "move into" vs "enter"), or add a "once per round" or "once per turn" limit on the power (which is slightly more restrictive than the second option if somebody gets thrown into the gateway several times by separate effects).
EDIT:
here's the argument thread about Storm Pillar, which also brings up Grease as having a similar problem: it's a zone with a free action triggered by someone entering it. On a hit, you knock the target prone. On a miss, you slide it 2 squares - which means you can slide it out of the zone and back in, triggering the free action again, and just keep going until you hit. Exact same question as Planar Gateway, really: obviously the DM should just say, "No, that doesn't work" as soon as you try it, but is it just moving someone out and back in that should not trigger it, or should no forced movement at all trigger it? (Just occurred to me: another way to forbid this is to add "in a straight line" after the shift effects, which makes sense for Grease especially.)
And
here's my post about the Illusion powers, which lists some nerfs and a probably way too long discussion of Illusory Wall (which got no replies at all, pout). Rereading that, I think the new wording of Illusory Wall is highly ambiguous, and I like my suggestion in the last paragraph for a fix. (Which is not a nerf, it actually makes the power stronger than the AP version.)