I've heard reports that many precincts around the country only enforced the ban on campaign T-shirts/buttons/other items among Republican voters. A good friend of mine (an African-American woman, and a rabid Bush supporter) reported seeing several black voters in line at her polling place wearing Kerry buttons, and one guy was wearing a Kerry hat. The precinct workers made no effort to tell them to remove those items, despite the fact that Ohio has the same prohibition on displaying campaign items inside a polling place. Of course, the area my friend votes in is heavily Democratic, in a county Kerry carried by 1% (Montgomery County, Ohio).
In Greene County next door to Montgomery, I stood behind a woman with a "W 04" shirt. I was not offended.