Saw a note on MacGamer's Ledge revealing that the clamor for the Quake III Arena hats and mousepads that are being handed out on the Bus Tour has been heard, and these items will soon be for sale. Here's the scoop from the id Quake 3 Arena page:

A bit of cheerie news for all of you who have been e-mailing about the Quake III Arena t-shirts, hats and mousepads. Your voices have been heard. I've received news that we will soon be making them available for purchase via 1-800-id-games. I'll make an update on these items once they become available.