Flickr users are annoyed they need to go through Yahoo's log-in to get to their site and have threatened mass suicide if the policy is not changed. Susan Crawford thinks the problem is trust: you may be a good company now, but what might you do later? Same for Google.
I think the Flickr users are right to be concerned. Yahoo and Google have valid business reasons to try to tie their users closely to the site with mandatory single log-ins in the same way Microsoft packaged programs with the operating system. They're trying to create one-stop shops that make it hard for users to pick and choose the best products across vendors (or make it too easy for users to stick with a sub-par but easy to access product).
Susan's correct to note that here, more is at stake than user choice-- it raises a major privacy concern. The types of services provided under one-login-- email, message boards, shopping, calender, now pictures-- are where the wealth of PII on the Internet lives. They are your digital identity, and do you really want all that information held by one-vendor with no laws in place that regulate how they can aggregate and use PII collected from their various applications? Do you really want to rely on privacy policies for this? Or, as Susan asks, are we ready to simply trust them to "do no evil?"
This might not be exactly what the Flickr fans are worried about, but it is what they should be worried about.
BTW: I've gone through dozens of log-ins with Yahoo. At log-in, they ask for all sorts of PII that you can fake, but they make you give your birthdate as a security question if you lose your password. I've lost mine many times since I don't use it that often, and cannot remember the fake birthdate I gave them so I have to get a new log-on. This cannot be the way we protect privacy online moving forward.
UPDATE: Mary Hodder's discussion of how this relates to the larger conversation about identity management is really excellent.