Why dose this happen??? I’ve asked this question before but only now I have a pic.http://img184.echo.cx/my.php?image=why27mo.png
in edit mode, with all the faces selected: CTRL + N in order to recalculate the normals. it should solve your case.
anyway, when you have faces like this, you don’t need to smooth them: in the Editing buttons (F9 key), I suggest you to press the Set Solid button instead of the Set Smooth button…
hope this helps,