The $499 Surface model appears to be a popular item on the Microsoft store's Web site.
As of this afternoon, Pacific Time, that model (without the Touch Cover) is listed as "out of stock." Previously, it , along with the $599 and $699 models.
The other two models -- both come with the Touch Cover -- are no longer listed as back-ordered.
"The store periodically will run out of certain models," a Microsoft representative told CNET in response to an e-mail query.
Calls to Microsoft stores in Century City, Calif., Newark, Del., and Scottsdale, Ariz., revealed that those stores had stock of all three models.
However, the Glendale, Calif., Holiday Day Store (a kiosk) was out of the $499 model and a representative at the Scottsdale store said stock was running low on the $499 model.
And note that Microsoft gave away thousands of the 32GB Surface models with the Touch Cover at the Build conference this week to developers. It's not clear if that has had any impact on inventory.
The Surface RT tablet is Redmond's first branded PC and sales are being by PC makers like Acer, who are trying to gauge when -- and if -- to bring out products based on Windows RT.
Surface is being sold only at Microsoft retail stores and at the company's online store.