When a nether portal block is in the player's inventory when a world is loaded (through /give, and inventory editor, etc.), it will render as a solid cube, with 6 sides each showing the nether portal texture. The portal can be given in game as long as it is not placed. As soon as the portal block is placed the block in the inventory begins rendering normally, and any additional portal blocks obtained will render correctly. The game must be completely shut down in order for the portal blocks to render incorrectly again. If a portal block already exists in the world, the portal in the inventory will render normally.
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Confirmed.

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
(Automatic Message)
Confirmed in 08a with /give @p portal
Confirmed for 14w10c
No longer obtainable using /give command.
Confirmed.