Ok, what with the downtime and general state of being constantly busy, I lost track of this thread for a while... and it got a little ugly. Let's keep this civil and friendly, folks.
That said, can't we just agree that if a power/effect/whatever causes an infinite loop or an auto-hit situation, that it's wrong and you should use common sense? In the case of grease, I think it's pretty obvious that sliding out and back in again shouldn't trigger a second attack -- but if you slide him out, and your fighter uses Tide of Iron to push him back in, that does trigger a second attack. I don't remember how Planar Gateway works, but surely there's some other common-sense interpretation we can come up with.
I do think that there's a possibility for a reasonable case where a single power's zone effect could reasonably affect a single target more than once in a turn, even without cheesy slide effects. Here's a situation I've seen come up in combat: Wizard casts Wall of Fire in a zig-zag, then uses Thunderlance to push an enemy in such a way that it enters and leaves the wall several times:
Code:
| ABCDEFGHIJKL
===============
1| F F
2| WBF F F F
3| F F
F is the wall of fire, B is the bad guy, and W is the wizard. If W manages to push the bad guy 5 squares east, he'll enter the wall of fire three times.
Does he take damage each time? I say yes. To me, this is absolutely classic wizard play.