Some developers from the Spanish blog Ataraeo have created a lens for browsing YouTube videos from the Unity Dash. To do so you have to install a YouTube Scope and a Video Lens. Here is how to install them on Ubuntu 11.10 Oneiric Ocelot.
Open a terminal and type
sudo add-apt-repository ppa:atareao/lensessudo apt-get updatesudo apt-get install lens-video scope-youtube
Log out and log back in. Now click on the Dash and select the YouTube lens. Search for the video you want. Results can filtered by Category, Relevance, Publish Date, Viewcount and Rating.
Once you click on a video your favourite browser will open and the video will load in YouTube.
It would be nice if the video opened on a standalone application such as Minitube or UMPlayer but I believe the developers might do this in a feature version.
Update: The developer has released an updated version of this lens. Now when you click on a video it automatically opens in VLC. No need to open the browser. Nice!
Update 2: Support for Minitube was also added. Open dash and type scope-youtube-configurator. Now you can choose how you want the YouTube videos to open.