Is there a REAL performance difference between INT and VARCHAR primary keys?

Is there a measurable performance difference between using INT vs. VARCHAR as a primary key in MySQL? I'd like to use VARCHAR as the primary key for reference lists (think US States, Country Codes) and a coworker won't budge on the INT AUTO_INCREMENT as a primary key for all tables. My argument, as detailed [here][1], is that the performance difference between INT and VARCHAR is negligible, since every INT foreign key reference will require a JOIN to make sense of the reference, a VARCHAR key will directly present the information. So, does anyone have experience with this particular use-case and the performance concerns associated with it? [1]: http://database-programmer.blogspot.com/2008/01/database-skills-sane-approach-to.html#rule1