YouTube saw 20.7 million unique visitors in September, according to comScore. But YouTube had over 30 million streamers, suggesting that a good number of the YouTube videos are viewed elsewhere, like on MySpace. MySpace could block the YouTube videos (though MySpacers may not be too happy). News Corp could also sue YouTube and distract them (though that action has its share of risks).
But clearly there is leverage on News Corp's side. How will such an ad-sharing deal work out? I'm not sure, but News Corp is in a good position to ask for a good chunk.