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.