Minecraft wiki entry for World_boundary, associated text: The world boundary, at 30,000,000 meters from the center of the map. Notice that vegetation is cut off at this point.
A redstone torch hanging on the invisible wall in Release 1.7. This can be achieved by placing the torch on a non-solid block (such as glass).
In Release 1.7, slimes try to go to Z:30,000,000. Notice the abnormally large redstone dust.
A village generated near the world boundary.
Lighting does not work past the world boundary.
Torches placed on glowstone next to the invisible wall, the particles emitted from them are also in the wrong place.