Here's a guide for museums on the care and documentation, including proper shipping practices, of ivory antiquities: http://www.connectingtocollections....oint-Slides.pdf
In my discussions with US Fish & Wildlife, importing pre-Act ivory should go smoothly, but there are a few pitfalls. He said that the most common pitfall is pre-export permit validation. For example, if you're importing ivory from the UK to the US, you must have the UK customs authority validate the permit and sign off on the export. If it arrives in the US with all of the proper documentation, etc., but the permit was not validated, then it will be treated as an unpermitted import.
Sounds like a common sense step, but it is frequently missed.