My rule  is - if you are not shaking hands with the hiring decision maker and introducing yourself, then you need a cover letter to introduce you.  If you are shaking hands with the hiring manager, you are introducing yourself verbally and requesting an interview.  In this case, hand them your resume without a cover letter. 

On the other hand, if you are sending a resume by mail, if you give your resume to a friend to hand in, or if you leave your resume with the hiring manager's secretary then you are not shaking the hiring manager's hand and you need to have a cover letter enclosed with your resume.