The change on the official guidelines comes from a tech writer noticing that I normally say "we work hard to prevent this" and asking back in February 2012 if we should update the language on that page. I said yes and the language changed in March 2012. It looks like this article is from April 2012.
One of the big reasons we say softer things (rather than "it's impossible") regarding the idea of negative SEO is that we have seen people to pretty crazy things to steal/hijack domain names in the past, like the bizarre history of sex.com: http://en.wikipedia.org/wiki/Sex.com
People talk about negative SEO far more than people actually attempt it, because you're never quite sure what effect (say) pointing some links to a site might have--it might help the site instead of hurting it--plus it's typically a better use of your time to develop your own site.
But if there's a site that is worried about negative SEO, the site can disavow any links they want using our the disavow tool in Google's Webmaster Tools. You can even sort to see the most recent links if you're worried that this is something that just started.
The primary usage of the disavow tool is so that a site can disavow bad linking that the site did itself and can't get removed from the web, but the disavow tool also works fine to disavow links that you're worried might be spammy.