When a user signs up for a new blog, he's asked to choose a user name and enter his email address. Then he chooses Gimme a Blog and goes to the next screen. There, his username is already populating the Blog Domain field. If he doesn't remove it and put in his desired blog name, his domain will be username.bigblog.com, not myblog.bigblog.com.
This seems like a very confusing thing to me and my new users. I don't know how to reword the signup code without breaking something.
Also, when you sign up, you're told there will be another email containing the generated password, but the password appears in the browser, no 2nd email needed. Confusing.