Arrows (And possibly some other types of entities) will not pass through nether portals in 14w31a.
I changed the world to 1.7 to see if it only affected the 1.8 snapshot, and it does. Arrows pass through portals for 1.7, not 14w31a.

(Other entities may also not be able to pass through nether portals.)

Edit: when I say "pass through", I meant like the arrow will not go to the nether