What's the appropriate HTTP status code to return if a user tries logging in with an incorrect username / password, but correct format?

The correct HTTP code would actually be 401. From the RFC:

The 401 (Unauthorized) status code indicates that the request has not been applied because it lacks valid authentication credentials for the target resource. The server generating a 401 response MUST send a WWW-Authenticate header field (Section 4.1) containing at least one challenge applicable to the target resource.

If the request included authentication credentials, then the 401 response indicates that authorization has been refused for those credentials. The user agent MAY repeat the request with a new or replaced Authorization header field (Section 4.2).

401 - Unauthorized

403 - Forbidden

http://www.buggybread.com/2012/11/http-error-codes-401-access-denied-403.html