One of the few truly interesting articles on Smashing Magazine about optimizing pngs for their filters. It shows that yes, the filters are really a huge part of how pngs get compressed:

http://www.smashingmagazine.com/2009/07/15/clever-png-optimi...

No, I wouldn't say that. Those filters make a huge difference to the compression ratio!

The png optimizers you see (optipng, pngcrush, etc)? What they do is carefully pick the optimum filters to get the best file size. So I'd say those filters were the most important part of the compression.