I'm giving some thought on how to do it and prototyping in SP. Or at least I was until I got into a fight with a pig-man and I lost all my obsidian, glass, grilled pork and diamond tools.
I'm hesitant to being though. We have to figure out the intricacies of these portals before they all get planted about willy-nilly.
Note:
Never, EVER pick a fight with a pig-man. I'm pretty sure they're all on the same team, so if one gets pissed off, they all get pissed off.
Wherever you build the your gate is not where you come back. It duplicates your gate for some reason.
Not much of a problem in MP, but watch out anyway: http://games.on.net/article/10630/Minecraft_Update_Includes_Inventory_Destroying_Bug