Last modified: 2010-12-06 00:54:37 UTC
Since the Wikipedia logo (http://commons.wikimedia.org/wiki/File:Wikipedia-logo-v2-en.png) was modified from a PNG-8 version to a PNG-24 version on May 20, it is no longer transparent in Internet Explorer 6. PNG-8 has transparency support in IE6, but PNG-24 images require a CSS or JS hack. There is supposed to be a hack for this at http://en.wikipedia.org/skins-1.5/common/IEFixes.js but it doesn't seem to be working. This bug is only for applying a CSS/JS hack to the logo, but is similar to bugs that ask the fix to be applied site wide.
This particular problem was fixed by a reupload in June, see Bug 2074 for a request for better IE transparency in general.
The file uploaded in June did not actually resolve the issue, it just maked it less visible. The background color was modified to match the vector skin, but it was not perfect. There is supposed to be a gradient at the top, but instead the logo appeared in a grey box. That being said, it would appear someone fixed the code, as I can no longer see the grey box. The logo now works with all skins and even Wikipedia projects where the PNG background was not modified appear fixed.