-If you build one at the bottom of the ocean, you might not be able to light the portal on fire.
-I'm guessing that Notch will make it so if you make a portal in the real world and it appears above lava in Hell, then it will make a small floating piece of land in the lava or something. And I'm guessing it would do the same if you make a portal in Hell and it appears above an ocean.
Speaking of teleporting into oceans, we will also be able to teleport into vaults and bases very easily (down-side).
Maybe he could make it so that you must place a portal in the real world, and hell world (on the same x/y/z coordinates), to be able to travel between the two worlds. Which would mean that he would first have to add some pre-made portals scattered throughout the world at first. And perhaps the portal will not open until there is 2 portals on the same x/y/z coordinates. This would defeat the purpose of super-fast transport though, and would have it's defects so I'm not sure about this, but its all I can think of for now lol.
Portals will most likely only appear where you create them in the real world..
Notch has confirmed that once a portal is created in the realw orld a new one will be made in hell in the corresponding spot. The only way to be portalled into a mob spawner or under the ocean is if you intentionally made a portal there. You can't tele into vaults or bases very easily unless you've already made it in, and the owner doesnt notice the enormous portal.
That might not be entirley true... in new chunks of the world there might be portals generated, plus I slightly remember notch saying something about there beeing randomly generated portals... not 100% sure on that though.