Considering how heavily Google has been weighting its own properties in the SERPs lately, I think this is an interesting move by Twitter. What I think will be really interesting is how other search engines, particularly Bing, take advantage of this.
From Search Engine Land, Twitter’s New SEO Play: A Profile Directory Of All User Accounts
With the new directory, Twitter is finishing that process by making its user profiles more discoverable by Google, Bing and other search engines. When asked why Twitter launched the user directory, a company spokesperson said it’s “to help people find the accounts they’re looking for with various search engines.”