When PNG8 is not the right choice
Dave Artz has a nice post about PNG8 and alpha transparency, my extensive comments on his article are awaiting moderation right now 
Dave has a very cool blog btw, only a few posts but they are all really good, there's a video tutorial of AOL's Pagetest tool and a discussion on non-blocking javascripts, something I just talked about over at the YUI blog.
(Illustration by Dave)
