Greg, the outlet you’re thinking of is ColoradoBlvd.net, a local paper here in Pasadena, CA, which does support webmentions including backfeed of interactions with Twitter using Brid.gy. (Sadly Facebook’s API turned off their access to this sort of feature on August 1st.)
As for Ben Keith’s concern about spam, yes, Webmention can be a potential vector like trackbacks and pingbacks, but it does learn from their mistakes with better mitigation and verification. Work on the Vouch protocol/extension of Webmention continues to mitigate against these issues. I’ll also note that Akismet for WordPress works relatively well for Webmentions too, though there have still yet to be examples of Webmention spam in the wild.
For publishers using WordPress, there are some excellent plugins including Webmention (which has some experimental Vouch plumbing included already) and Symantic Linkbacks which work with WordPress’s native comments. I’ll note that they’re developed and actively maintained by several, including the core maintainer for pingbacks and trackbacks in WordPress.
I’m happy to help if anyone has questions.