Apparently AMD used to give a lot of these out at events. Doesn't seem to tbe the case anymore.
I can vouch for that. I checked with their booth at Microsoft Tech. Ed. 2004. Nothing but an AMD pen and sticker. However, I did get an AMD flash drive for test driving the Visual Studio beta.
I really like the idea of marketing through Polo/T-shirts. It's highly visible and like Pfosse mentioned, what better way to develop credibility than to see the company "IT" guy with an Athlon 64 shirt.