FYI. The reason for this price drop is that Sigma just announced a new version of this lens. It is a new design. 9 elements instead of 7. It looks like improved AF (one of the only negatives you read in reviews of the current lens is that the AF can be spotty) is the main thing. Also, closer minimum focus distance and thus greater maximum magnification. There is also a new USB connectivity feature that allows you to make some adjustments to the lens. I dont know if this is available on any other lens yet, and it would require buying a dock of some sort. How much this adds is questionable to me at this point.
That said, its still a great deal and Im considering pulling the trigger. The new lens will probably be at least the msrp of the current one which is 70% higher. A hefty premium.