Proggie: Privacy is something we take very seriously at Flickr. People have actually always been searchable by email address on Flickr (via "People Search"), and we chose to respect our member's existing people search preference settings (as well as make that preference page easier to find).
On the opt-out versus opt-in issue, this is something where we carefully weighed the options, and chose the default option based on what we feel would have the greatest benefit for the majority of our members. Just as we chose to make "public" the default for uploaded photos, we chose to preserve opt-out as the default for people search on Flickr.