YouTube will soon be dropping support for Internet Explorer 6 as it attempts to move users to a 'more modern browser'. Google, who owns YouTube, is pushing for users to upgrade to Chrome, Firefox or IE 8. It's no surprise that they are pushing for users to switch to Chrome with two advertisements to download the browser which Google is using to challenge Microsoft's IE 8.
YouTube so far hasn't officially communicated about the desire to drop support for IE6, but it's conceivable that like Digg it would rather have its developers spend time optimizing the service for newer, better browsers than wasting man hours on the oft-despised Microsoft browser.