http://www.youtube.com/t/terms and are unambiguous:
Section 4: General Use of the Service—Permissions and Restrictions
Part C You agree not to access Content through any technology or means other than the video playback pages of the Service itself, the Embeddable Player, or other explicitly authorized means YouTube may designate.
Section 5: Your Use of Content
Part B Content is provided to you AS IS. You may access Content for your information and personal use solely as intended through the provided functionality of the Service and as permitted under these Terms of Service. You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content.