The iMac is said to be in short supply, suggesting that a refresh may be imminent.
Availability at US stores, including Best Buy, is said to have dried up. According to AppleInsider, the Best Buy website lists the iMac as "not available".
AppleInsider suggests that the limited availability could be due to the Back to School promotions Apple is currently running.
The most recent refresh of the iMac was in May 2011 and the last time it saw a significant redesign was 2009.
A launch of a new iMac is feasible this month, perhaps in conjunction with the launch of OS X Mountain Lion, which is anticipated to launch on 25 July, following Apple's Financial Results announcement the day before.
It would seem likely that a new model will offer Intel's latest Ivy Bridge-based i5 and i7 processors. Rumours also suggest that an anti-glare display may be available.
Apple is also rumoured to be working on a new iMac with Retina Display, but the launch is said not to be likely before October.