so does the end portal spawn them or does the chunk in the nether with the portal stay loaded so the pigmen have to walk through?
i'd like to point out that this also means you can herd angry pigmen into the overworld.
End Portal in The End is bugged right now. I did some testing, and End Portal seems to send everything but the player back to The End's spawn point. So until they fix it, the only entities that can return from The End are players. Everything else teleports from the Ender Dragon's portal to the End spawn point.
In terms of moving between dimensions, it does not spawn new entities. Entities can move between dimensions. I was messing around and thought I witnessed a skeleton I put through a Nether Portal come back into the Nether without me in the chunk, but it might have been another one that walked through. Usually, when you go back to the other side, the guys are all stacked up there, so it doesn't keep those chunks loaded. It might load the chunk for a moment so they can move out of the way, though... Not sure.
Ender Eyes as keys: http://www.youtube.com/watch?v=hE9ytwDYG-YWhy don't I see this more often in adventure maps? It's been possible for a long time now. Block update with End Portal Frame.
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