With the last iMac update having arrived more than a year ago, third-party resellers are now starting to see limited availability of Apple's big-screen 27-inch desktop Mac.

As of Wednesday, AppleInsider noticed stock-outs of the 3.7-gigahertz 27-inch iMac at Best Buy and J&R, while only a handful of units remain available at Amazon, where it is advertised that more are on the way (Update: Amazon's listing no longer indicates limited stock). In addition, MacMall is currently sold out of the 27-inch iMac with a 2.7 gigahertz processor.

The lack of availability at Best Buy is particularly noteworthy as the retailer's website simply states the all-in-one computer is "not available" both online and in store, with no indication of a restock. In contrast, Apple's newly released MacBook Pro with Retina display is advertised as "backordered," with new orders expected to ship between 7 and 30 days from Best Buy.

Limited availability at Mac resellers is often one of the first signs that Apple is drawing down inventory ahead of a product update. And the iMac line is due for an update, with the most recent refresh released over a year ago, in early May of 2011.

However, it should also be noted that Apple's iMac desktops are a popular choice for education institutions. Last month, Apple kicked off its annual back to school promotion, offering a $100 iTunes gift card with the purchase of a new Mac, which could explain limited availability of the 27-inch iMac.

Just this week, a new report out of the Far East claimed that Apple's supply chain is currently gearing up to begin production of the next-generation iMac. However, that report suggested the updated all-in-one desktop won't become available until October.

The new iMacs are expected to feature Intel's latest-generation Ivy Bridge processors. Apple's MacBook Pro and MacBook Air lineups were upgraded to Ivy Bridge with a product refresh last month.

It remains unclear whether the next-generation iMacs will feature Retina displays like Apple's new MacBook Pro. Reports from earlier this year suggested the new iMacs would in fact have new high-resolution displays, though more recently doubt has been cast that the addition will make the cut for Apple's 2012 models.