Microsoft executives are likely breathing a sign of relief today. Just one day after opening up pre-orders for Surface RT, the company has sold out of its least expensive 32GB model.

Microsoft's Surface pre-order page now states that the $500 tablet, which does not come with a Touch Cover, will ship "within 3 weeks." The other two Surface RT options, both of which come bundled with a black Touch Cover, still promise an Oct. 26 delivery date.

Given that the Touch Cover is a major selling point of Surface RT, it's a little odd that the model that doesn't ship with one would be first to sell out. It's possible that sales were driven people who want a cover in a color other than black, and are buying both separately. It's also possible that Microsoft intentionally kept supply for the $500 model lower than the higher-end models. It's unclear how many of this specific model Microsoft made available for pre-order. And the company isn't spilling any beans, other than to let us know that more will be available.

When asked for the specific number of units sold, a Microsoft spokesperson said, "No numbers to share, the $499 one is now on backorder so that means that SKU is out but all three SKUs will be ready for launch day."

Both higher-end Surface RTs, which come with the much-admired Touch Cover, are still available on the Microsoft store, indicating that demand for the device has not reached a crazed iPhone- or iPad-like level. And consumers who want to choose the color of their Touch Cover might still opt to wait for the $500 Surface RT to return to sales channels.

In any case, the sold out model is a good sign for Microsoft, given that the Surface RT is its first foray into building its own tablet. It's too soon to know if it's a hit, but at the very least it isn't a dud.