So Warner Music isn't as petty as I thought it was. According to this story in Billboard, Warner didn't ask for YouTube to remove videos of the recent Led Zeppelin show from YouTube. Rather, it was a company called GrayZone, which has been authorized to issue takedown notices on behalf of Warner. In this case, GrayZone acted on presumption, and YouTube's automated system inaccurately attributed the notices to Warner.
This makes an interesting point: under the Digital Millennium Copyright Act, the copyright owner is responsible for policing each violation and requesting its takedown. (I'm not a lawyer, but the relevant section in the act seems to be: "The burden of providing written notice of design protection shall be on the owner of the design.") So it's not enough for Warner to say "remove all videos of any band signed to Warner" or even "remove all videos of Led Zeppelin's December 10 concert." Rather, they have to point out each copyrighted work individually. That's a lot of work, which is why they've apparently outsourced the job to GrayZone.
No word on whether YouTube will automatically restore the videos, but I think not--users probably will have to repost them. And of course it's still possible that another party--the show's promoters, the owner of the venue, the band, or its management, even Warner (for real this time)--might ask for them to be taken down. In the meantime, LiveLeak has a few videos (thanks to Wired's Listening Post and Salon's Machinist blogs for the tip).