I brought a bag of it back through Boston IIRC, about 15 years ago. It wasn't a problem and I'm sure they could smell it through the two layers of plastic bags and my backpack. Had a lot of fun with my hardfiskur - I had to keep it in a big glass jar so the housemates wouldn't be too grossed out by it, but it was a great treat every now and then with a bit of butter as discussed elsewhere.

But, perhaps the customs person just took pity on younger-and-dumber me, who had been out on the town in Reykjavik for about two days, trying to keep up with an Icelandic friend showing off the night life there...