Terraria developer Andrew 'Redigit' Spinks has posted on the Terraria Online forums announcing official tees available for pre-order via a partnership with Split Reason! There are three designs to choose from to start, at $19.95 per shirt. With enough interest, more designs may be released in the future, so be sure to post your support in the thread!
Find out everything there is to know about the game at the Official Terraria Wiki, including a list of confirmed future additions to the game.
Terraria developers often post small teasers and discussions on the Terraria Online forums under the names Redigit and Tiy, so for the most current updates from them be sure to follow their activity there! You can also find guides, maps, server listings and everything you need to make your Terraria experience complete.