You can teleport out of worldborder with /tp but dont place blocks with /setblock

Confirmed for 14w21b

But probably intended, at least that you can't place blocks

Intended for both.

As of 14w20, you are now able to fly freely outside of the worldborder (gamemode 1 or 3).

As of 14w18, you are no longer able to place/remove blocks outside of said border.