I'm not a graphics enthusiast but I seem to remember reading that the png format supported things that gif doesn't offer, like 32 bit true color and semi-transparency. I think there's a bit more to it than licensing issues. I did a quick read through the faq on the png site
which looks like a good stop for anyone wanting some real details.
Hey... it even looks like Oreilly has a book out on the png format if you're into that sort of thing.
"So... What do all these little arrows mean?"