Sorry, I guess I should have asked for clarification. When you asked if the "lens you mentioned" had been release, there had been 3 lenses metioned... the Sigma 50, the Sigma 30, and the Nikon 50. So... which lens are you asking about? Only the Nikon 50 is a recent release... the other two have been out for a while, which is why I assumed the Nikon was what you were asking about.
Yes. The primary difference is that the older is an "AF" lens as opposed to an "AF-S". The older lens requires a body that has a "focus motor" built into the body to power the autofocus portion of the lens. The new AF-S version has all of the necessary components to do autofocus within the lens, so it will work on all Nikon bodies regardless of whether the body has an internal AF motor or not. The only bodies to date that do NOT have the motor are the D40, D40X, and D60. I believe this is the complete list to date that will not autofocus with the older AF lens version.
Also, the new version is a "G" which means there is no external aperture ring. Aperture must be set with the camera. It cannot be set on the lens. This is of no consequence unless you plan to use the lens with an older (non digital) body. It does not matter on any Nikon digital body.
There are other differences, but those are the main ones. The new AF-S lens focuses faster than the older version and is quieter. I have an AF and an AF-S. I will be selling the AF. In the two examples that I have, the newer lens is MUCH sharper wide open than the old both in the center and especially in the corners. It is KILLER sharp across the full frame on a D3 by f2. It also gives better contrast than the older AF lens. I have compared my two on both the D60 and D3.
Thank you. Do you have any fotos posted on flickr, etc where you used this lens? I ned to check out both the slightly older AFS lens and this one to make sure I don't get them confused...I can do that easily.