This website gives several examples of cover letters. They typically begin:
December 1, 2010
Miriam A. Nunberg
Staff Attorney
U.S. Department of Education, Office for Civil Rights
32 Old Slip, 26th Fl.
New York, NY 10005-2500
Dear Ms. Nunberg:
If you do not know the hiring manager's name and use a generic address, how would you change this format? Would you exclude the name and position and only write the company's address?