Upon loading a previously-created world in 1.8.1 pre3 for the first time, I found that a previously-created pair of Nether Portals were placing me about one block forward and a few blocks to the side of my expected exit location. On both ends, this placed me in a wall.

To further test, I mined out one block of the Overworld-side portal and reignited it, to find that the problems persisted. The other portal pair I had previously created also displayed the issue. However, a new portal pair specifically created as a test placed me where expected.