This is a followup to the following meta post:
We have been in contact with Matt Cutts regarding removing nofollow:
... 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.
Starting today we will be removing nofollow on links within posts that hit a high enough threshold to be considered reputable. The details will remain somewhat vague at the moment to discourage gaming of this feature.