Probably "Preparing to ship" now means they're ready, but will be delivered Dec 27, which seems to be some sort of embargo date. (It's probably also when reviews will go up.)
They might even have been able to assemble some of the
non-Recommended Configurations in that time too.
the trash can - that did eventually turn up in 2013 and luckily my delivery went exactly as expected, fingers crossed it's the same this time around)
You were lucky: I was watching it go live on Dec 19, 2013 (in Australia) and even though it was a comfortable 6 or 7 pm locally, I was dithering around deciding on core count, D500 or D700, etc, and didn't place an order for a few hours.
By that time, the est. delivery date had hit the end of February. After giving it a few weeks, I eventually gave up waiting, canceled, and built a hackintosh.
(Which I survived with through Mavericks and driver update hell - still have nightmares about getting 4K 60Hz to work - then relented and bought a fully loaded 8c Mac Pro anyway.)
NOW of course I have a complete "beat the rush" system worked out, consisting of site-primed Mac, iPhone with iOS Apple Store app, and a VPN on hand in case Apple's CDN starts playing favourites with who they give early access to (a la iPhone X). Got my iMac Pro ordered by 2 mins past release this time.
The pain of missing that Mac Pro 2013 order cut deep!