"Unfortunately unforeseen worldwide demand for several of our lenses, the 18-35mm, 24-105mm and 150-600mm have caused the slow delivery times. If you could be patient the lens will be delivered but you will really need to check with the dealer you ordered the lens from to better determine your actual delivery date."
Few weeks ago I received a report that all 150-600mm lenses were pulled back due to some issues found after the first batch was shipped. If this is not the case and Sigma USA is correct, people on the pre-order list should still be receiving the lens in limited quantities. I have not seen any reports online of anyone getting the lens since the end of December. Please feel free to prove me wrong in the comments section (if you have received the new Sigma 150-600mm lens in January).
"Sigma doesn't have production schedule of this lens until the end of February and not sure the schedule after March. If they will have production schedule in March, it would be shipped out in the end of March. But, if they won't... it would take at least another one month."
@DafEOwen CP+ Camera & Photo Exhibit takes place in Japan Feb 12-15, any new products are likely to be announced then.
— Sigma Imaging UK (@SigmaImagingUK) January 22, 2015