Apple's explanation is that machines come from plants all over the US and more than one plant in China. These different facilities do not have the same stock/availability of parts (eg. 7200 RPM drives) nor are they equally fast at processing orders (some are slower than others, do less volume than others, etc.). Therefore orders placed with Apple are not shipped according to who ordered first, but rather, your order gets routed to a plant via some algorithm (unknown), and once your order is queued at a plant, it is fullfilled FIFO there (at that plant).
Apple tries to take all of this into account when they give out estimated shipping dates, but it's not a perfect overall estimate.
The people who take your order at Apple's online store have no way to knowing where your order is, though you could perhaps ask for a manager and press the issue.
Obviously, it's frustrating to hear claims that people who ordered after you did have already received their merchandise, whereas you have not.
I ordered June 15th, 15" 2.2 MHz matte, 7200 RPM, have not received anything and still is not being processed. I did receive the free printer within two days, however.
Oh well...