I'm getting two different impressions, here.
My clear (if incorrect) understanding is that the 3-year (add'l) US warranty is only available when buying through a Sigma USA authorized dealer. The Sigma Web site lists the online and bricks-and-mortar dealers so authorized. Cameta is on that list as of 31 October 2008. See
Sigma Corporation of America.
So you should get the
Sigma extended warranty when buying through Cameta. Now ... what proof is needed for repair is a different story. I did get a card with my 10-20 purchased from, I think, Adorama. Maybe B&H. I would like more than an email from a friendly customer service rep but Sigma has a fine reputation so perhaps that's OK.