Dave Shea recently detailed the technical benefits of using the PNG image format in Web projects, noting, however, that even the most recent versions of Internet Explorer do not support alpha transparency, the most compelling reason to use the format. As a result, some argue that the only reason to consider PNG is to support the spirit of open standards. Despite basic browser support for PNG, we continue to use the GIF format on this site, which I guess means we have drawn the conclusion that there is currently no overwhelming benefit to switching formats.
We've signed the "Proper PNG Support in Internet Explorer for Windows Petition to the Microsoft Internet Explorer for Windows Development Team." You should too. With your help the situation may be better three years from now. Read Jeffrey Zeldman's comments on the PNG petition if you're unsure why you should sign.