So I clicked on one of the example invite links listed in the bug report. I was then prompted to log in to my Yahoo account. Hmm, OK... so I logged in, at which point I was taken to a page with a form asking me to join Flickr. I did NOT fill in or submit the form because I don't want a Flickr account, though it was presented with defaults based on my Yahoo ID anyway. I closed that window and tried clicking on an invite link again. Much to my surprise, the link then worked and I received an email saying "Welcome to Flickr". What the hell?

[edit: on the plus side, Flickr make it very easy to delete your account entirely. The only obvious side effect is that the screen name you had is now unavailable for any further users]