It is potentially endless; however, falling into the Void causes the player's health to be depleted by about 4 hearts per second, leading to the player's death in 2.5 seconds or less. Even in Creative Mode the Player still dies. The only way to get out of the void is to quickly fly out of it on creative.
Entities (Mobs and Items, excluding the player) entering The Void will be deleted in the following tick (as soon as possible). This may be due to The Void not being part of a Chunk, although it is most likely a deliberate optimization/crash prevention method implemented by Notch.
This has been as of 11w49a. Mobs will fall through The Void until it reached Y=-64 at which point they receive the same damage as the player (4 hearts per second). Items will fall until they reach Y=-64 where they disappear.
Blocks cannot be built on the underside of the Bedrock layer, and although Water and Lava appear to flow into it, they will not actually spread and form beneath the lowest layer. This prevents players from descending into The Void safely via swimming.
There is currently a bug in Survival Multiplayer that will remove chunks client-side, so while the data is still there, the client cannot see it and will glitch if they step on it. This can be fixed by reconnecting or placing a Torch next to the missing chunk. If you ride a Minecart into this missing chunk, and then you step out of the Minecart , you will fall to the void and die, regardless if you disconnect and reconnect. Light cannot travel out, though, or into the void, treating it as an eventual black-hole.
The Void's color used to be blue, but is now black. If you use Night Vision effect, it will show as white.