I found this at   
http://www.erowid.org/ask/ask.cgi?ID=2693
So, the situation, as I understand it:
1.  It is illegal to sell thujone containing absinthe in the US for human consumption. 
2.  It is illegal for someone outside the US to sell thujone containing absinthe to someone inside the US. 
3.  It is not illegal to purchase thujone containing absinthe for personal use in the US. 
4.  It is not illegal to purchase thujone containing absinthe for personal use from outside the United States 
5.  Thujone containing absinthe can be seized by US customs (if it appears to be for human consumption).