In the weeks heading into Macworld, both Apple Stores and Apple Retailers are reportedly running low on stock of Apple's iMac. In contrast, Apple's online store still shows shipping "within 24-hours".
The significance of the low iMac stock is unclear, but could simply be due to recovering inventories after the holiday rush. Of all the Mac models, the iMac seems least likely to undergo a revision at the upcoming Macworld Expo in San Francisco.
The iMac was last updated in August of 2007 with a new aluminum design.
Update: We've heard that this was simple due to the holiday rush and that stores are restocking.