No rebates for 300mm f2.8 IS II? 
I was able to get 10% off my recent 300mm f/2.8 IS II at Focus Camera. Really. After talking to them (post-order) , it turns out they are an old NYC camera shop (with good BBB rating), that are trying to get with it in terms of their website so they are/were offering 10% off all orders for first-time customers. Not sure they're still doing it, but if so, just check out their site,
www.focuscamera.com and sign up for the email list when the little window pops up on your first visit. They'll email you a promo code. Other than having to wait a few days for them get one from Canon USA and shipping ($35) it was a helluva deal. It was a USA warranty, too, with case, and all. Totally legit.