When you have public profiles, checking for presence of an account is pretty easy anyway.
Right. You can also check for someone's profile with his name or email on Twitter (or even just Google them).
"Those options correspond to however you’ve authenticated with the site in the past. In my case choosing “Twitter” redirects me to Twitter for the standard auth dialog, and picking “Bagcheck” displays a standard password field."
As wgx suggests, it does look like the auto complete shows two Sacha user accounts. I presume this is before you get "those options to choose your authentication method".
Right, but by typing a few characters I can establish if a user has an account or not, for a subsequent attack.