Material with alpha channel set MaterialBase.requiresBlending to true, but DefaultRenderer set depth test to false for blended renderables.
Compare planes with alpha http://dl.dropbox.com/u/1036911/away3d4/alpha_planes.swf and solid planes http://dl.dropbox.com/u/1036911/away3d4/solid_planes.swf you can hack this behavior, and turn on depth test for blended renderables here http://code.google.com/p/away3d/source/browse/trunk/broomstick/Away3D/src/away3d/core/render/DefaultRenderer.as#101 But it's dangerous, and better wait until that suggest the authors.
