Viacom are trying to salvage their reputation in the YouTube information hand-over by denying they want to infringe your privacy:
They want user names and IP addresses, and the videos which people have watched. This, of course, is information which could be completely anonymous, but - as we saw with the release of supposedly anonymous data from AOL - there is anonymous data, and data which can be pieced together with other information to not be anonymous any more.
On a broader philosophical note, why is someone's Google ID not an identity which should be treated with the same respect as your name in the real world? Many people use the same screen-name in many places; if you've done nothing illegal, and haven't been accused of anything illegal, shouldn't that identity be spared from being violated by a corporation in the same way as your passport-and-social-security identity?