Google has upgraded its mobile version of YouTube so that it shares more in common with the desktop version on HTML5 enabled phones. This anticipates the idea that more people will watch videos on the move than on the desktop, so you'll get a better browsing experience, the ability to like and dislike and other social elements.
Put this in the context of a Jupiter report which says there will be 300 million mobile broadband subscribers by 2015 and you can see why Google would want to do something like this.
More importantly it's arguably taking the mobile video market into its own hands just as it did the desktop video arena. Most of the mobile phone companies started with, if they didn't continue with, their own proprietary video standards. Smartphones such as Android - OK, that's owned by Google so of course it carries YouTube - and the Apple iPhone work with YouTube rather than their own technologies.
This move tells developers they need to continue to integrate and be prepared to accept video feeds from outside, perhaps within their own applications. It should also be telling video producers doing marketing and other corporate exercises through YouTube that it's going to be viewed on a much smaller screen than before - so no high-def whizzbangs, for the foreseeable future a simple picture is going to make a powerful difference.