A two week old post on the Terraria Online forums has word on plans to update the 2D sandbox RPG, including the possibility of Steam achievements and an OS X port (thanks Ant via GamingOnLinux/Linux Today). To manage expectations, they note they are not definitively promising either of these, in case something goes wrong, but a later post asking about Linux support adds this to the list of things they are working on, with the Creative Director Cenx saying: "Once he is done with the mac port he will be looking at Linux." As for when, they say there is not even a suggestion of a release date at this point. Here is the list of what's in the works, where the last item points to a screenshot of the game running under OS X:

It has been a while since we gave any update on the things we have been working on so I felt it was a good time to get you guys up to speed on things! I will try and keep it short and to the point...

• As many of you know we have been working on a new forum as well as a revamped website for Terraria! Crowno and Skiphs have done an amazing job with both of these things! The website is done and the forums are getting very close! We anticipate it being done within the next 2 months. (Trying to throw out a safe timeline.)
• Yoraiz0r has started looking into Steamworks to get achievements added to the game. Once he has added the first one successfully this will be put on hold until the end of the update. This will give us time to think of the achievements we want and get all the assets needed for them. *There is always a chance for disaster so this is not me making a promise for achievements!*
• All of the assets are done for one of the new events and most of the tedious programming has been finished. All that is left for this is programming the boss, setting values, and setting the drops.
• Crowno made a pretty awesome concept for the Lunar Boss that we are going to go with rather than using the falling moon idea.
• Oh and Skiphs has been working on this for the last few days...