In the same boat here...
Ordered on 10/30... i7/8gb/512gb ssd/295x -- expedited ship
Went from prepping to ship on 11/7, back to 'processing' on 11/10.
I called apple on Monday morning, and after about 10min on hold, the agent came back and said that the only time this ever happens -- where a BTO steps backwards in the queue -- is when there is a part(s) shortage. Furthermore, the rep said they throw the status one notch in reverse to give customers the option of canceling the order and sourcing a machine elsewhere, as once the BTO enters "preparing for shipment" the cancellation option is no longer available.
Since both the 5K imac and the regular 27" imac use almost all of the same components, aside from the screen & the video card, the cause for the delay most likely lies with the screen itself... as if you hit the applestore online and try to BTO a regular iMac, they currently have a 2-3 business day lead time, where as both versions of the 5K iMac (290x vs. 295x) are showing the extended lead time. So... by simple deduction, that leads me to believe that either Apple ran short on displays, or they ran short on BOTH video adapters for the 5K... my bet would be on the displays themselves.
Based on what i've read here.. the last known orders that actually shipped were placed on 10/28. Looks like we just missed the last of them.
Here's to hoping the parts come in today, and they shift production into high gear. I for one, sold my previous 27" iMac almost 10 days ago, in order to help fund this one. I need a large screen to edit on.
I beg you apple, with a cherry on top... change my order to Shipped!