Do we even need a sign-up dialog? Couldn't we just log in, and if it doesn't exist yet, tell the user to verify via the incoming email?

I'll grant that there's likely some stuff that some sites want to do on first visit, and that if some do it, users will eventually expect all to be like it. And perhaps that's why we have two dialogs now. But is there any security/pain point related to it? (Assume there's some small snippet of text that makes it clear what's going on, I guess.)