The Legendary and CE's are going to be hard to find day-1 because there's a limited printing of both. I know one or the other is limited to *just* 500,000 copies.
I overheard a GS/EB employee (when I was put on "hold" to ask about Blue Dragon coming in) telling a person that if they want Halo3, that they have to pre-order. He then stated that they have over 500 preorders at this little rinky-dink EB/GS (Again, at this one, they had 20 preorders for Forza, and around 80 for Oblivion for X360 and PC).
There are 2 reasons they are stressing preorders:
#1. It makes them far more money - Think about it, if you give them $5 for a pre-order, your investing that cash into them at a 0% APY. Extra profit for them.
#2. MS really doesn't know the demand outside of pre-orders. Also, there's no way to really solidify where the extra non-PO copies are going to go. I have a feeling that each store will probably get around 20-30 non-preorder copies per store. That's ALOT for a normal game (20-30 per general store would be probably around 200,000-300,000 unsold units).
In order for MS to properly saturate the market, it'd have to undertake somewhere around 750 copies PER STORE on AVERAGE. That's vastly too many units. Why? Imagine what would happen to Halo3 if it sold beyond 1,000 units at a store that otherwise wouldnt of sold much, whilst another store (a big one, lets say) gets 750 copies and sells 50 on launch day. That would be a disaster for the distribution centers since they would have tens of thousands of USD $ products lying around, taking weeks to be transfered to stores that need them.
So it's better for them to not have too many non-preordered copies (again, in the 200,000-500,000 area) in stores, with another 500,000-1,000,000 in distribution centers waiting to ship more units to the store as demand requires.
The videogame market is unique in this aspect of sales, as there really isn't a market out there that has the same issue of uber-first week sales, and massive shortages.
And finally....If you were the distributors of Halo3, what would you rather do: buy 100,000 copies that are already paid for at $60 each (on average, wholesale), or buy 250,000 copies with half of them unsold - That'd be a $9,000,000 purchase of unsold goods. And with any game, you never know if it's going to sell or not (think ET).
I think MS, EB/GS, and the other places are doing the right thing by stressing preorders. Lets just hope that whomever goes out there to buy a non-PO copy finds one.
What I'm REALLY wondering is: if Halo3 sells 2m to 3m units first week, where will Week 2 be?