Upload & Sell: Off
Usually they are offered to the seller and some sellers pass them on to the customers. You'll find a lot of bitter commentary on the subject.
Usually this rebate period coincides with a price increase from Canon, so when the lower price inventory sells out, the price goes up and you get MORE vitriolic commentary concerning vendors "keeping" the rebate.
So you're left deciding whether you'd rather sell your lens now while people are shopping, or wait till January when perhaps they've bought a new one.