Don’t Squander Your Brand by Using Many Domains for the Same Site
Link Equity and Authority Consolidation : SEO Book.com
I agree with this concept but disagree with the example. Digg.com gets so very many backlinks that splitting their brand isn’t really hurting them. Digg is in the link business and does things like incenting people to link their blog to a digg ref rather than the news article. This gives them a good bit of authority and IMHO allows them to ignore “link authority” considerations and focus on what they think will be best from a human user standpoint. That’s the right focus to have I think.