MacNN is reporting that some Apple retailers are running low on 27-inch iMacs. The shortages seem to be limited to large chains and discount sites, such as Best Buy and Amazon; Apple stores appear to have stock available.
Courtesy of Apple
Stock shortages and delayed shipping times are early indicators of new Mac models entering the supply chain. We've been expecting new or updated iMacs this year, with fall the most likely time for them to make their debut.
Coincidentally, Apple is rumored to be holding a September 12th press event to announce the iPhone 5 and an iPad mini. A new iMac doesn't really fit into a press event designed for mobile devices, but if it's just a basic upgrade, such as a faster processor, then it may be announced with little fanfare.
I'd rather see lots of fanfare, with lots of new features and changes to the iMac, but I think we're more likely to see the current design get a speed bump.