This isn't new, and it's still dead in the water. On September 14, 2011, 58 US Senators (45 Republicans and 13 Democrats) have expressed their opposition. Considering only 1 in 3 senators need to vote "No", it's unlikely to pass.
Also, the Arms Treaty doesn't exist yet. It's all reports and committee meetings. Even if it did, it would not trump the Constitution. Worst case is it could restrict imports/exports. US arms industry is over $40b per year in exports. Not so much rifles or pistols, but tanks, helicopters, etc.
http://www.un.org/disarmament/ATT/documents/Pretty much the justification for thinking the ATT is a bad idea:
http://www.controlarms.org/homeAs others have said, treaties don't trump the Constitution. But they are accorded, theoretically, above domestic law. Sorta, kinda, not really. The reality is, we either sign the treaty and ignore it, or pass laws to implement it ourselves. This is not always good. There is a Harmonized Commodity Description and Coding System (HS) of tariff nomenclature that we signed onto. Basically, it means imports/exports get identified to a specific code, which makes life easier for friggin everyone. So, naturally, we created a Harmonized Tariff Schedule of the United States which is mostly the same. But you have extra annoying stuff. But, in fairness, it's better than Schedule B nomenclature, so...
If we sign a treaty and don't enforce it, it's on the books but no one cares. CBP gets stuck with most of that enforcement, because most treaties deal with either international travel or commerce. Few treaties deal solely with internal laws. Which sucks, because CBP has to enforce a couple hundred different laws and treaties. You have to be REALLY book smart to implement it. I like to think I'm fairly bright and my head was hurting just dealing with international arms trafficking. Don't go into CBP's Customs enforcement sections unless you are REALLY bright and can handle implementing hundreds of Byzantine laws. Most CBP agents can't, hence the high turnover and morale problems. On the flip side, you'd make very good bank for the rest of your life as an import/export specialist, especially if you got your Customs Broker license.
So, in short, it's not going to pass. If it would pass, the economic implications to our domestic "arms" manufacturers would be more of a concern than to individual firearm owners. The US is one of the largest arms manufacturers of the world, that would be a legitimate concern. But dude, Raytheon, Boeing and LockMart own more Senators than the unions and the NRA put together. Why do you think we're willing to spend ridiculous amounts of cash on military aircraft? I would not be surprised if those three companies alone had a "marketing" budget larger than the gross revenues of every firearms manufacturer in the entire US.