It appears that YouTube will be phasing out support for Microsoft's still-popular-but-insecure Internet Explorer 6 browser. Banners have started appearing for IE6 users that say "We will be phasing out support for your browser soon".
A number of upgrades to "more modern browsers" are suggested, including Google's own Chrome, Microsoft's IE8 and Mozilla's Firefox 3.5. Chrome gets a second link along the side of the page, too.
There hasn't been an official announcement from Google yet about the changes, but fellow Web 2.0 success story Digg recently also vowed to cut support for the browser, due to the inordinate amount of development time it consumes. Digg blogged that IE6 users make up 5% of site traffic, but only 1% of usage.
However, a survey that the site conducted revealed that the vast majority of IE6 users - 80% - want to upgrade, but can't as they're stuck on corporate PCs that don't have an upgrade policy, or they lack administrator access.