This is IH from isohunt.com. I've went through this same process with our lawsuit brought by the MPAA so I thought I'll share some of my thoughts on the issue.

As unreasonable as I think these lawsuits by Viacom/MPAA may be, in order for the legal process to work, plaintiffs are entitled to evidence in order to prove their case. However, user privacy should be a large concern in disclosing of data (logs) as evidence, and in neither Youtube's or our case, there's no reason for turning over data that would expose your personal identify (such as your IP address). From glancing the order against Youtube, the reason they were ordered to turn over user histories is to prove user infringements, and inclusion of IP's in such logs is to uniquely identify users who may have signed up multiple usernames/accounts. I call bullshit on that. If someone uses multiple usernames, he can as easily login with multiple IP addresses, disclosing IP's would not help the plaintiffs in proving copyright infringements. I expect Google/Youtube to appeal the order (at least I sure hope so).

As for us, we successfully argued in our MPAA case that we don't need to turn over your IP addresses as it is a violation of user privacy with no evidentiary value, and only turned over .torrent access logs in anonymized form. You may not like to hear that .torrent logs are being turned over, but the truth is we were ordered to do so and that the MPAA does need anonymous logs to prove their frivolous lawsuit.

More at http://isohunt.com/forum/viewtopic.php?t=134054