Ah. I have a pet peeve with Adobe's products in that regard. You'd expect that the world's premier raster/vector/layout/etc editors would flawlessly handle images they support. Hah, how naive of me!

Whenever I save a PNG in PS I always have to crush it. I've found advpng to be better than pngcrush in general, but both preserve alpha. Generally I get about 20% savings over PS.

The visual differences you were noticing though, are you sure that wasn't due to color profiles and gamma?