Update 6/4/08: The deal has been announced, as reported below.

We’ve got confirmation that CBS is going to announce an important partnership with Yahoo tomorrow morning.

While we don’t yet know exactly what it will entail, we are confident that Yahoo will be joining the CBS Audience Network, which distributes CBS content to destinations such as YouTube, AOL, MSN, Joost, Veoh, Fancast, Bebo, and TVGuide. See a longer list of partners here.

The CBS Audience Network launched in August 2007 and streams 70,000+ videos to over 300 partners. More than 190 million streams have been delivered since launch, the most popular of which are highlighted on the network’s own destination site.

Videos are generally delivered with pre-roll advertisements, and revenue is split between CBS and the partner websites. In addition to syndicating to other sites (but, notably, not Hulu), CBS offers video content through its main site and iTunes. The company recently began testing high definition streaming as well.

Yahoo TV already provides content from NBC and FOX (via Hulu), making the addition of CBS a trio of sorts. We hear CBS will cite the inclusion of Yahoo in its network as providing it with an unprecedented 92% US online reach. It also appears that CBS’s collection will be the single largest content contribution to Yahoo TV.

While YouTube joined the CBS Audience Network quite a long time ago, it only offers short clips without any pre-roll advertising. Therefore, it lacks the full-length content found elsewhere in the network, and presumably starting on Yahoo tomorrow. With all the activity on the web surrounding the monetization of full-length content, and the recent departure of its head of monetization, one wonders whether YouTube should start taking long format, professional content more seriously. Or start to lose some of its luster to competitors like Yahoo.