Google says that all pre-orders for the standalone 8 GB version of the tablet have already shipped, while pre-orders for the 16 GB Nexus 7 placed until July 11 will ship this week. Any pre-orders for the 16 GB version placed after that time will ship next week.
Here's Google's explanation for customers in the United States:
We’ve shipped all standalone Nexus 7 8GB orders (e.g.: those without a case, charger or Nexus Q). By the end of day on July 19 (PDT), we will have shipped all standalone Nexus 7 16GB orders placed through July 11 (PDT), and upgraded these orders to overnight shipping. We will process the remaining standalone Nexus 7 16GB orders by the end of next week with overnight shipping.
If you ordered your tablet with a case, charger or Nexus Q, your Nexus 7 will ship this week with overnight shipping, in some cases ahead of the rest of your order. But don’t worry, the rest of your order will be on its way soon.
The company also says it has received "incredible demand" for the new tablet. Indeed, the new tablet has reportedly sold out in most stores already.