dcollins
Explorer
Here's a WOTC "Rules of the Game" article which asserts that you cannot move diagonally across wall corners, but (unfortunately, imo) you can move diagonally past enemies: http://www.wizards.com/default.asp?x=dnd/rg/20040713a
Corners: Corners, such as where two walls meet at a right angle, block movement. You cannot move diagonally past them, though you can trace line of effects for attacks past them. When you attack past a corner, you foe has cover. Most squares that contain foes are impassible to you, but you can move diagonally past them and your target doesn't get cover if you attack along that diagonal. This is because creatures don't completely fill all the space they occupy on the battlefield.