As soon as you enter the nether, the game creates a new portal in the nether, outside of the (nether)worldborder (see screenshot1) and the player is trapped.

• You cannot destroy or place blocks
• If you teleport yourself back within the (nether)worldborder, you cannot destroy or place any blocks either
• If you go back through that portal to the overworld, the game creates a new portal next to the worldborder (see screenshot2)
• same effect in SSP
• Bug does not appear in snapshot 14w33c and before (tested it)

The worldborder in this world is centered around -3330x & -8200z and set to 970, moving outwards at a rate of 10 blocks/day.

The fix:
set the worldborder to 10000 or above
(but then there is no point in using it at all)
Effect of the fix:
the nether portal (overworld) connects perfectly to the pre-placed portal in the nether which position can be calculated.