Only when I recently compared the firmware revision of my Nikon D90 with other older camera that I realized there's an additional "L" version number! All "A", "B" and "L" are currently at 1.00. Probing deeper on the Internet revealed however that this firmware contains the invaluable lens information needed to optimize its use on the Nikon D90!
No wonder when I first picked up to shoot with a Sigma lens, all hell broke loose without one single accurately focused image. And according to the Internet, some blokes who bought newer lens (e.g. Nikon 35mm F/1.8) found it difficult to get crisp images via the Nikon D90.