I'm pretty new at Minecraft, so I finally built my first portals just the other day.
I have one at sea level near my spawn point, and on a whim I built one down in a large cavern where I get my lava/obsidian (lava level?). I stepped into the lava level portal and ended up in the portal in the nether that was established when I used my first portal at sea level.
I then stepped back into the portal from the nether side and ended up at my sea level portal. I've repeated the experience, so I can use the portal at lava level to get to sea level via the nether, but I can't do the opposite.
Is it impossible?
1 block in nether = 8 blocks in real world. So if you travel 24 blocks in the real world, you're only 3 blocks away in the nether. If you're LESS than 8 blocks in the real world (X/Y axis, Z doesn't count) its the same block in the nether.
Now, I'm not 100% sure of the exact numbers, but I suspect Notch's code looks for a nearby, existing portal in the nether to connect to, rather than creating a new one, when you go through a portal in the real world. It also does the same in reverse, and spits you out at the nearest one it finds.
So if you have two portals in the real world connecting to the same 1 in the nether, it'll always spit you out in the real world at the same spot.
So when you go into the first portal in the real world, it spits you out where you first landed in the nether. Then, when you go into the 2nd portal in the real world, it spits you out at that same portal, because in the nether, its relatively close.
When you exit the nether, your first portal is the closest one it finds to where you 'should' exit at, so you'll always exit there.
The way it works it that if the area in the Nether where you are attempting to portal to is blocked, the portal relocates, and when you step back into the portal, you end up in the relocated area in the real world; in the future Notch will probably fix this.
Nether is a Dimension right?
Therefore the opposite world.
Therefore down is up, and up is down.
Up is more mining, but down is normal.
Physics are normal.
So therefore Nether is a Mirror world.
Up and Down have same generation. (I think)