This is called the "Black Tear," and is meant to be a super-secure base (whether for PVP or just any survival play). It is made entirely of obsidian, and functions well when placed high-up in the sky with little to no dynamic land or structures in the surrounding area (i.e. in a desert or ocean biome). Furthermore, it offers the most security on a server with factions enabled, or in any other situation in which the owner is exclusively able to teleport into the structure, since entry into the tear is operated from within the tear itself.
The only entrance into the tear is found on the very bottom: a 2x2 hole. When closed, piston heads block entry and are protected by a short "collar" of obsidian to prevent explosions from reaching up into the entrance hole. When the piston heads are retracted, which is done by standing on a pressure plate in the main chamber of the tear, water is released which falls to the ground (or ocean) and forms a water ladder up into the base. This allows other players to ascend into the tear with your permission. If you are in a situation in which teleporting is unavailable, the pressure plate can be exchanged for a lever so that the entrance can be left open by the player. This severely reduces security, but perhaps you could find creative ways around this. The water which falls from the tear also allows for movement within the tear (there are three levels inside; the bottom is largest, the top is smallest).
Knowing that explosions reach through "corners" between blocks, I reinforced the main chamber in such a way that no explosions can reach the interior. Therefore, the main chamber can safely be used as a storage room without the fear of the player's storage chests being blown-up. The top two levels are not secure in this way, but can still be utilized (I used the second level as a furnace room; there's not a lot at stake with furnaces being vulnerable).
This was my PVP base on the SMP server "VindiCraft" during Beta 1.8.1. Mining all of this obsidian took quite some time, but the protection was worth it.

Additional Notes

Let's make SMP PVP obsolete! Everybody turtle into impregnable obsidian shells and taunt other players into coming out of their impregnable obsidian shells so that you can laugh at them for not being able to get into your impregnable obsidian shell!