Observation today (9/22/07) around my area (we have Target, TRU, Bestbuy, Walmart and FM)
TRU has several (6 or so) of Halo 3 360 and 4 of the Premium. No core and Elite is also sold out
Best Buy - 2 Premium and 1 Halo 3. No Elite. This is a very small Best Buy and they usually don't have a lot of stock.
Wal-Mart - 2 Elite, 2 Premiums and 2 Halo3. They probably have more in the back but that's what I saw in the case. Again, no Core.
Target - 1 Elite, 2 Halo3 and 2 Premium.
FM (Fred Meyers) - 1 Premium. No elite (they had one but it was sold). They are a lot smaller than Target and Walmart so they don't really keep big stock. They still have 20GB PS3 from Christmas ....
So I don't see any shortage at least no in my area. I don't really know how well they sold, but it seems there are plenty of the 360s of any Sku with exception of the Core.
I was thinking that Halo 3 360 would be hard to find but it seems there are plenty out there.
