Whitey - 1:24 pm on May 12, 2013 (gmt 0)
@indyank / JesterMagic - These references wouldn't be permitted here, but there's a lot of commentary on major SEO blogs and some studys. Put a few keywords into search and plenty should come up, including the "must do" lists.
There was also a press release here on a study which triggered a lot of follow up articles : [prweb.com...]
I've no idea of the accuracy of the information, but the giste of articles is that Google has a trend for lowering it's tolerances and guidance on dumbing down spammy link related issues, both internal and external.