I just cancelled my B&H preorder (good news if you preordered with them as I had placed my preorder within the first hour) after I found out that I could get 6% total cashback when using my discover card at J&R.
The 'shop discover' program is well known at this point, but basically you can order your 5dIII from J&R (the only affiliated retailer that you can preorder it from, although you may be able to preorder it later on from retailers like buy.com, circuitcity.com, compusa.com, etc.) for $3500 with free shipping and no tax (unless you live in NY) and get 6% cashback (5% from the J&R deal and 1% from your normal cashback deal with discover) if you use your discover card (and you have to login to your discover account at shopdiscover.com and go to jr.com using the link there).
So, it breaks down to $3500 - $210 = $3290 total. It pained me to cancel my B&H preorder (and I wish I had thought about this when I initially preordered -- J&R told me today that they had "hundreds" of 5dIII preorders at this point, so it could be a substantial wait) but for $210 cashback it was worth it to me. If it's worth it to you also, you should take advantage of this offer also (and no, I don't work for either J&R or Discover).
Btw, I did confirm with J&R today that the cashback offer applies to 5dIII preorders for the full amount (no upper limit on the cashback amount).