The amount of theories, opinions, rumors and articles about the Xbox360 launch and more specifically the amount of stock is growing with the minute. Here I try to give you some kinda of overview.

From kotaku.com, joystiq.com, shanghaidaily.com, spong.com, ve3d.ign.com:

* Electronics Boutiques: Store managers at EB are calling customers to bump them off of the first allocation of Xbox 360 pre-orders.
EB Games is facing severe shortages of Xbox 360 stock according to one employee who wrote in. The employee stated that his Arizona store has had over 90 pre-orders, 32 of which were guaranteed to be fulfilled, until today when the EBG head office revealed that the store would receive just 16 units on November 22nd, leaving another 16 "guaranteed" pre-orderers bitterly disappointed, and another 60-odd sod out of luck for who knows how long.
The company as a whole will supposedly only receive around 35,000 (VE3d source) / 40,000 (Kotaku source) its to distribute to all outlets across North America. Total pre-order is around 75,000 (Kotaku source)

* GameStop: Kotaku reports 18 at one store, despite 90 pre-orders. VE3d on the other hand reports that GS employee told them that everything's fine and they will be able to fulfill all preorders.

* Wal-Mart's site tells people to go to a store at midnight on November 22, the day Xbox is released in the United States. A Wal-Mart employee broke down how that fine chain would be handling the big day:
-All stores are supposed to open at midnight to sell their limited supply of 360s and games.
-Specific store's 360 supply will be dictated by corporate based on how their game sales are in general.
-Store supplies will likely be similar to what Target stores will be getting. (10 to 50)
-Only one 360 can be sold per a transaction.

* Target: 10 to 50 Xbox 360s depending on average volume of store. Every store will print up tickets representing the number of 360's and core systems they have in stock. Starting at 6:00 a.m., employees are supposed to go out and hand out tickets to people in line on a first come first serve basis.

* Best Buy: 20 to 60 Xbox 360s with a larger shipment coming "later that week." They decided to handle it like the PSP launch, handing out tickets at the door to line-waiters.

* Amazon.com Inc says the system is unavailable in the United States and warned UK buyers they may not get it by the holidays. Bids are as high as US$4,100 on EBay Inc, 10 times the suggested price of as much as US$399.99.

* Research analyst Mike Wallace predicted that only 1.5 million Xbox 360s will ship this calendar year, with less than a third of those (400,000) available in the United States on launch day.

* Opinion Spong: Nose-tappers are claiming that Microsoft is trying to organise what has been described as a ‘Holiday Stampede' for 360 by keeping shipped-through hardware from retail at peak periods and snatching headlines in the process.
SPOnG thinks this highly unlikely. Microsoft would much rather be announcing strong sales over the coming busy period than high, unfulfilled demand.

* Opinion JoyStiq1: Conspiracy theory? It just makes too much sense for Microsoft to play the product launch like this. Heck, Sony did it with the PSP by talking about Beanie Baby / Cabbage Patch Doll shortages during their launch party for the PSP (turns out, there was no shortage after all).
The bottom line: if people were rational, you'd be able to walk into just about any store on November 22nd and obtain an Xbox 360. But all it takes is a little, good, old-fashioned FUD (Fear, Uncertainty and Doubt) to drive sales through the roof and make this rumor a self-fulfilling prophecy.

* Opinion JoyStiq2: This is super-savvy marketing. Selling a product isn't always about creating enough to fill immediate demand. It's about sustaining demand over the long term. If this launch is played out correctly, long-term sales of the console should prove stronger than the typical console launch.
If someone has run the numbers on this, and if it can be determined that lower supply will result in higher long-term sales, they why not do it?
Crackpot conspiracy theories aside, maybe those Xbox 360 factories just had some glitches and didn't produce as many units as expected. That's the official line from Microsoft.