At X/0, Y/0, and Z/0 is the Void. Typing /tp 0 0 0 in chat will bring you the Void because Z/0 = 1 block below the last layer of bedrock.
At X/12,550,820, the terrain is severely distorted. One side effect of this distortion is that the map shakes while the player is moving. This distorted land was called the "Far Lands". It was removed in Beta 1.8.
At X/12,550,820 and Z/12,550,820, you will travel to the corner of the map. It's called the Corner Far Lands. Once again, removed from Beta 1.8.
At X/30,000,000, something unexpected happens. Instead of this being the end of the world, chunks are generated, but walking on them will cause you to fall into the Void and be killed. Instead, using Creative flying will allow you to "walk" on these so-called "fake chunks", and you can dip below them to see cave systems, lava lakes, and maybe even abandoned mineshafts, dungeons, or even strongholds. No mobs spawn here.
At X/32,000,000, this is commonly talked by Minecraft users as the "end of the world" but we will get to the the hard limit later, and as of snapshot 12w25a, is the farthest region of Minecraft that can be accessed without mods. Fake chunks generate, and entering this boundary will cause the world to crash and be "disabled". A disabled world always has the X/Z coordinates at this boundary, and the player cannot move any direction.
At X/2,147,483,648, no more chunks appear to generate. Going about 1 block further will cause the game to crash. Thus this is the maximum amount of world the player may access in-game (without mods).
At X/34,359,738,368, no more chunks generate anymore. Due to the game crashing at X/2,147,483,649, this boundary is pretty much impossible to reach.
I experienced something that made me come here. I was exploring a creative server and everywhere I went unnamed peices of work by people somewhere in the world, of which I will never meet. It was a sense of just how small you really are. It was that feeling when your alone for hours and feel separated and lost. I never want to travel that road again. Led alone 2,000,000,000 blocks away
Apparently someone did this legitimately, I think it took him 5 days straight? To reach 12 million x.
That was a while ago though. And I wish they never removed the farlands, because they were awesome :<
False. Strongholds only generate "between 640 and 1152 blocks from the origin, 0/0", and there are only 3 of them. That means the closest you can be to a stronghold once you reach the boundary is, assuming the stronghold generates at exactly (0,1152) and you take a peek at (0, 30000000), you'll have to see 29,998,848 blocks away. Pretty sure at some point between those two points, the stronghold chunk will become inactive and stop rendering.