The main reason to use PNG over GIF from a legal standpoint is covered here:
http://www.cloanto.com/users/mcb/19950127giflzw.html
The patents have apparently expired as of 2004, but the idea that you can use PNG as open-source over GIF is appealing to many people.
(png open source reference: http://www.linuxtoday.com/news_story.php3?ltsn=1999-09-09-021-04-PS)
