Michael Tree said:
Political assasinations would happen using scrying, teleportation, and a signle true-strike spell. No world leader would last more than a couple of months.
Doubtful. Political leaders will have access to 10 times the magical resources of the average assassin. Can you imagine a dozen secret-service wizards in black robes and pointy hats, with wires in their ears, flying alongside the presiden't car?

[/B]
Better yet:
screen. It's right next to
scrying in the PHB, and completely roots it. I'm sure someone will figure out how to bind it to a car. I've used
screen against a party that can teleport, and they still haven't figured out what's happening. So it's 8th level - intelligence services do all their recruiting from universities, don't they? (Well, most of it, I guess.) My point: An area that's been
screened is almost teleport-proof. You really can set up a magical fortress if you want to, proof against anything but a good old-fashioned dungeon crawl by the assassins. (Module WH-1: The White House. Sorry, bad taste.)
Now on the topic of extraplanar emigration: It wouldn't happen too fast. These other planes would either have border control already, or institute it really fast when they realised all these smelly humans were flooding in. Really, if you can fly, teleport at will, and break rocks by looking at them sternly, do you want these tiny tourists wandering around with cameras going, 'Ooh! Pretty!" and then dropping coke bottles in the Spring Of Eternal Beauty?
In addition, there's the whole extraplanar politics thing going on. Even amongst the good planes there's supposed to be rivalry, and the evil planes have the Blood War. Let's not even start trying to describe the internal politics of the Abyss. Now, in the real world, if you have a passport stamp from Israel you can't get into Indonesia (or so I hear). The outer planes would be similarly mistrusting of anyone who tried to get in, and if they've been somewhere else... whoo boy, I pity the tourist who tries to go from the celestial planes to a motel owned by Bowel-Tearer the Balor.
I think that the argument about extraplanar technology comes down to what Arthur C. Clarke said: Any sufficiently advanced technology is indistinguishable from magic. All outsiders have innate magical powers that no mortal can hope to emulate. That's an edge, right there. Outsiders will always be superior to the vast majority of prime planers. No matter the source of magic, the outsiders have it, and they probably don't need technology. Jet planes? No, I can teleport at will. Shotguns? According to the rules in the DMG, a balor
cannot be harmed by a point-blank shotgun blast that would tear most people in half. (3d6 damage vs. damage reduction 20 or so.) Is this because the balor is naturally tough from evolving in a pit of hellfire? Is it a genetically modified warrior race? Is it wearing a symboiotic cybernetic armour entity? Who cares? It can shrug off a shotgun blast, and that's all we know. Can it teleport by opening a paradimensional wormhole with a casimir device-type effect on a quantum level, or does it use magic to access the astral plane? Again, you don't know because it makes no difference.
I do like the idea of Secret Service Pointyheads, though. Ooh! Idea: Tyres Of The Zephyr. Like the horseshoes only for vehicles. That would screw up many equatorial economies. "We don't need to buy more rubber, our tyres don't suffer friction any more." One tiny thing makes all the difference...