I'd agree that h3 will probably manage a 50% attach ratio from mid October through December.
At my local EB/GS, in a mid-sized mall (town pop of 30,000), they have over 425 Halo 3 preorders.
Their breakdown?
120 Legendarys (sold out)
80 CEs
225 Normal Editions
A bit different ratio. However, the preorders are massive. Halo 2 only managed 250-300 at this store. So pre-orders are vastly above H2's levels at this point. However, having said this, they have advised me that they've stressed preorders - they said they are unlikely to have ANY copies of Halo3 in for pick-up and purchase near opening day. IMO, thats stupid, but oh well. Thats their business.
Back from the dead, I'm afraid.