This may not be fact, but was told to me when I ordered a Dreamcast before a friend of mine, and he got his and I was backordered. Take it with a grain of salt ...
Sometimes, the reason the item will list as still being in stock, is because at ONE of the many warehouses ... it is. However, when you order, it's automatically sent to the warehouse closest to you. The stock levels are not real-time, but they are close.
Problem is, when you call for status, they only check THAT warehouse. When you call, find out if they can move your order to another warehouse. Not every CSR will do this, but it's possible.
The Buy.com system is NOT FIFO. Don't assume just because someone ordered after you and got the item that you will as well. It's all based off when and to what warehouse your order was sent. Supposedly, this was to make Buy.com more efficient.