Here’s the full email from Matt Cutts, dated 11/3/2011
Hey Jeff, we’re trying to encourage sites with valuable user-generated content (like Stack Exchange) to have a more nuanced approach to the nofollow attribute on user-generated links. Using the attribute on all user-generated links takes away a big incentive from spammers, and prevents spammy links from being included in search ranking. However, good links can also be made invisible to search engines with this policy, so we miss out on that ranking signal, which could be used to surface better or fresher high-quality content that your users are recommending.
Would you be willing to try a slightly different approach? We’d like to recommend lifting the nofollow attribute on links posted by your most trusted users. So for example, right now it looks like all links in user comments are nofollowed. Since your readers are the type who could be posting links to resources that might be more obscure or hard to search for, it’d be great if we could be able to follow those links to use in discovery and ranking–if the user is high-quality. Links that we’d want to see could come from users who have been posting for a while with ever having their comments removed for spam, and users who use real names or nicknames (rather than names like “cheap furniture” or “web design”).
Let me know what you think, and hope you’re doing well,
Matt