There's usually an easier way to check if a username exists - skip the login form and try the signup form. If you can't sign up with that name because its taken, then it exists :)
Hence, signup forms tend to have CAPTCHAS more often than login forms. Also, it's easier to insert an artificial per-signup-form submission delay under the guise of "setting up your account". An artificial delay upon login might be annoying, but during signup it's not as painful (seeing as it's a one-time thing).