Hah, I was 4 minutes behind you with a white wi-fi 64GB and I'm still waiting for it to switch from "prepared to ship" to "shipped"!! I mean, the accessories are out the door already, but my iPad is languishing in China...*sigh* ...good things comes to those who wait...and I've been waiting! 😀
It seems like it's all just random order shipping, but that can't (or shouldn't) be the case. I'm not sure how Apple handles production lines over in China, but it wouldn't surprise me if each of the various models (two colors and three capacities in either a Wi-Fi or Wi-Fi + 3G, plus apply engraved or not engraved to all possible configurations) are assembled in different locations and perhaps in different factories all within proximity.
Also it probably depends on which models they allocated more resources towards or figured which iPad models would be the most popular (i.e. a 16GB Black Wi-Fi, just a guess). These iPads are being shipped straight from the factory as soon as they are assembled, but that still doesn't explain why someone ordering before me with the same configuration wouldn't have received a notification yet.
My only other thought is how the orders/transactions went through. Some credit cards could have taken longer to authorize the purchase for whatever reason causing a delay; like the unusual time of a big purchase in the AM, or some other reason I can't think of.
Maybe it just boils down to inefficient ordering or sales software. Who knows. Maybe since I used an

Gift Card I got bumped up in the production line, haha, I seriously doubt that though. I'm just speculating and throwing out possible reasons for the shipping notification delays.
Patients can be painful, and I really hope everyone who ordered in the early morning hours of the 11th will receive their iPads this upcoming week. Stay optimistic!
I'm leaving on a trip out of the country on the 19th and I'm very thankful that my order has progressed to where I should receive it before leaving. I think I speak for everyone here when I say we CAN'T WAIT to get our hands on our iPads. Especially if you're going to become a first time owner of an iPad like me.
In anticipation I've been buying some iPad apps and I've found a few on sale this weekend only. Here's a list to look at while you keep refreshing your order status.
All-the-ipad-apps-and-games-on-sale-right-now (via Gizmodo)