To make dirty transparency easier, I created a Photoshop Action that you can use to run on your PNGs.  Make sure to run it on single-layer PNGs that have already been saved off via Photoshop’s Save for Web.  Don’t try to run it on your multi-layer PSD file as the script is looking for a layer called “Layer 0″.

Download: Dirty Transparency Photoshop Action (1KB)

I also like his usage of posterization as a way of reducing colors.  At Ask.com, I developed a similar technique except using Photoshop’s color-indexing algorithms (perceptual, adaptive, etc) which I think offers the compressibility of posterization but does a better job at maintaining smoother gradients/detail.   Posterization’s diffusion characteristics are a little rough for my taste.  One of these days, I’ll post a tutorial on how we do this technique, which isn’t terribly innovative imho, but I haven’t seen anyone talking about it so it’ll be worth writing up at some point.

Meanwhile, be sure to check out punypng, as simply the best way to optimize your PNGs on the net.