It's another sign of the impending update to Apple's mobile phone: AT&T, the device's US carrier, has informed its retail staff to limit sales of the iPhone to customers.

Effective yesterday, customers can only purchase one phone at a time. This matches up well with moves around the world, where stock is either extremely low or depleted altogether.

Apple's European partner O2 has at points shown the iPhone completely out of stock, and Apple's online store shows the device as "currently unavailable." Such a statement is rare for the Cupertino company: typically it has ample stocks right up to the release of the new product.

"Effective immediately, customers can only purchase one iPhone device," the online advisory to AT&T store managers read.

To bolster stocks artificially, the company has apparently restarted a program to sell refurbished iPhones at substantial discounts. The 8GB and 16GB models will sell for $249 and $349 -- $150 off the retail price of the new models.

It is believed that the next generation iPhone will begin selling sometime in late June, roughly a year after the first model was launched. However, there are several clues pointing to a possible first appearance at the now sold-out Apple Worldwide Developers Conference.