Let's clear one thing up right now: NBC is NOT suing YouTube, nor have they joined Viacom in that company's litigation against the vid-sharing site.
Rather, NBC and Viacom have both filed a friend of the court brief on behalf of plaintiff Robert Tur, who sued YouTube in 2005 for hosting video copies of the beating of Reginald Denny during the LA riots of 1992.
According to the brief: "Many of NBCU's most valuable copyrighted works have been copied, performed and disseminated without authorization by YouTube and other similarly operated Web sites. NBCU has a strong interest in preserving the strength and viability of all of its legal rights and remedies in response to such conduct."
Viacom's role here is obvious. With their own $1 billion lawsuit filed, they're "all in" in a bet to sink YouTube. NBC's decision to file is more interesting -- the network has only worked more closely with YouTube in the days since it had "Lazy Sunday" removed back in early 2006. But this filing comes on the heels of their recently announced partnership with News Corp. to deploy their own video-sharing platform. Seems to me like they're hedging their bets on both sides. Or, as they would undoubtedly say, we're happy to distribute our content to every platform as long as we can control it, too. Dummies, the Web doesn't work that way.
In related news, the UK's Premier League sued YouTube on Friday, arguing both copyright infringement and inadequate tools to prevent that infringement.