I too love the T-shirt idea. Last week my team just had some T-shirts made, so we could catch some eyeballs on the streets while we hosted our walking-tours. We designed our T-shirts to have our company name inside a Search Bar and a mouse cursor pointing at the Search Button, hoping people on the streets will go home and google us!

I think its fair game to wander around and talk to vendors in the expo, but putting flyers on the chairs of a competitor's panel is kinda crossing the line.