Great rant on dialog writing from David Mamet to The Unit http://bit.ly/ba2xAA but it applies to everyone writing dialog.
(Caps from the quote)
THERE IS NO MAGIC FAIRY DUST WHICH WILL MAKE A BORING, USELESS, REDUNDANT, OR MERELY INFORMATIVE SCENE AFTER IT LEAVES YOUR TYPEWRITER. YOU THE WRITERS, ARE IN CHARGE OF MAKING SURE EVERY SCENEIS DRAMATIC.