[rogpow]

According to the Sigma lens catalogue:

"DG Lenses
These are high-performance lenses optimized for digital cameras.
They are ideal not only for 35 mm digital SLR cameras and film digital
SLR cameras but also for APS-C digital SLR cameras. They deliver
superior image quality by correcting for all types of aberration,
especially distortion, and they have high resolution capability because
they minimize chromatic aberration of magnification, which is
particularly a problem for digital cameras. What is more, these lenses
incorporate optical designs optimized for digital cameras and Sigma‘s own
Super Multi-Layer Coating technology, so they reduce flare and
ghosting due to reflection between the image sensor and lens
surfaces, and they provide high contrast and a wide range of tone.
They also provide enough peripheral brightness with little vignetting."

In their 2004 catalogue the lens in question is listed as:

180mm f3.5 APO Macro EX IF/HSM

by the time the 2006 catalogue came out, the listing is:

180mm f3.5 APO Macro EX DG/HSM

I've got several Sigma lenses from 12-24mm up to 800mm, all are non DG and all work fine on Nikon bodies.