Thanks to Julian K on dpReview for pointing this out... There's a list of affected lenses, and a pointer to finding your local Sigma service center... http://www.sigma-photo.co.jp/english/news/051221_hsm.htm To our valued customers, AF function of our HSM equipped lenses do not operate when "AF operation button" on the back of the body of Nikon "D200" digital SLR camera (recently introduced to the market by Nikon) is used. This problem is caused due to a firmware "bug" of our HSM equipped lenses. We deeply regret any inconvenience this may cause our customers who own D200 camera and we are committed to making this situation right. In this respect, we will start updating firmware of these lenses free of charge today. Please also note that if "shutter button" is used for AF operation of HSM equipped lenses, camera and lens combination works without any problem. This problem occurs with HSM equipped lenses only. Non-HSM models of Sigma lenses function properly with D200 camera.