This might just be me, but I interviewed with Google on-site about 10 months ago and never signed an NDA.
It wasn't even offered to me - and this was for an engineering job. So I wouldn't say it's 'standard', even at Google. That said, I've also never seen anyone blogging about their Google interviews who hasn't signed one, so maybe they just forgot the paperwork for me!
Are you sure? Do you remember entering your name on a touch screen in order to get your visitor badge? Yeah, there was a legal document in there. Probably should have read that.
It wasn't even offered to me - and this was for an engineering job. So I wouldn't say it's 'standard', even at Google. That said, I've also never seen anyone blogging about their Google interviews who hasn't signed one, so maybe they just forgot the paperwork for me!