Nikon does not share its technologies with lens manufacturers (so Sigma, Tamron et al have to reverse engineer their lens to get them to work with current Nikon bodies to get the lens to work). This is why on future Nikon bodies, the lens may not work. Nikon has not licensed their technology to Sigma etc
Hence even if the lens is an autofocus, it is very possible that it wont work on a newer body. I have the Tamron 90mm AF macro 1:1 (ie the gold graphics and Blue line around the barrell). I use it on a D70, and havent bothered to buy a newer body because it will probably not work. In a way, this is Nikon saying, buy our lens or risk not being able to use it on a future body.