Originally Posted by RawAlex

Incorrect. Part of Google's overall deal is where you link to. If you are linking to bad neighborhoods, your rank can suffer. If youtube was caught with tons of links to bad places, they could get penalized.

On the other side, if much of the supporting links for their PR come from places that have since lost PR, then they would suffer on PR as well. People who have used mass link spam techniques to build popularity for their own videos could also lead to Youtube losing a ton of PR.