You have the option of downloading and storing your favorite Partner videos and watching these videos even without an internet connection. To download Partner video:
1. Find the video you'd like to download.
2. Below the video's play bar in the lower left hand corner, you'll see a 'Download' button.
3. Click the 'Download' button to indicate that you'd like to download the video.
Videos are available to download in the MP4 format and some of them can be downloaded for free and they're even licensed as Public Domain or Creative Commons. An example of channel that offers Creative Commons-licensed video downloads is Stanford University. If the download requires payment, you'll be directed to a Google Checkout page where you'll find the price of the video.
Google Video had a similar option that has been discontinued in 2007, but it used DRM and it required to be online in order to authenticate your credentials.
Greasemonkey script for downloading YouTube videos
Update: YouTube's blog has more details. The downloading options are a test for US partners and there are 5 licenses for the downloaded videos:
* Personal, non-commercial use
* Creative Commons (Attribution-Noncommercial-No Derivative Works)
* Creative Commons (Attribution-Noncommercial-Share Alike)
* Creative Commons (Attribution)
* Public Domain (no copyright protection)