EDIT: Luke beat me to it
----------------------------------
I use png only when I need transparency. (Of course that means feeding IE another styleshhet and/or using the transparency filter). I still use jpg for the rest, probably out of habit, and maybe because I have more control over compression in PS.
Anyway here's a list with browsers and png compatibility
http://www.libpng.org/pub/png/pngapbr.html