So many have received them, I assume they are just an automated response set up by the warehousing system and likely related to the phone not being officially released yet. I'm not going to worry unless I haven't received a "shipping" email before April 6th.
On the other hand, if they sold through their ENTIRE inventory that fast, that's a good sign for the Lumia 900, Nokia, and WP7!
(And by the way, I ordered mine at about 9:30 AM Pacific, and I'm sure I was the first person in my store to do so.)