Simple trademark dispute. Presumably Icahn was smart enough to acquire all the trademarks along with the property. Who knows, maybe this was part of the negotiation that Icahn would get the property but not the trademarks and that is why they had to come to an arrangement (or Carl would have to change the name and we get new chips).

http://tmsearch.uspto.gov/bin/showfield?f=doc&state=4802:nmw0rg.2.29

Still seems to be live and still owned by The Donald. If you did a full trademark search there are many other places that use the name in different businesses not covered by his one (or many) trademarks.