If you're on your own, it's fairly obvious that what you wear and how you act make no tangible difference. However, when you have to interact with other people, what you do and how you act do make a difference, and what you wear is a part of that.

If I was invited round to a friend's house for a take-away pizza and a natter, it'd be counterproductive if I insisted on a plate, cutlery, napkin and a place at a table to eat the pizza. Ditto if, when invited to a black-tie dinner, I asked for my soup in a mug and a couple of slices of bread to make the Chateaubriand into a steak sandwich. Same thing with what I wear, since it can help or hinder what I'm trying to achieve.

If there are no expectations either way, my preference is for jeans. However, I'm not arrogant enough to wear what I want and believe everyone else should accept me as I am. As several previous posters have said, I'll wear what is appropriate for a given situation in the same way I'll adapt other areas of my behaviour to suit. And if an HR person believes Steve's T-shirt is inappropriate within the context of a relaxed, technology-focussed web site like SSC, they might find it useful to have another read through the textbooks related to their HR qualification; there are numerous references to how allowing people certain liberties can be used to achieve a given result (usually increasing productivity.