Apple usually quotes 24 hours for delivery of standard-configuration iMacs, but is currently showing 3-5 business days across the range. Increased shipping times are sometimes an indication of lower or sporadic stocks, which tend to precede the launch of new models. With WWDC just one week away, that would be a logical time to announce them …
It’s slim evidence, and may mean nothing more than random supply variations. If Apple does indeed have new iMacs in the pipeline, it’s likely these are a relatively modest update, in line with the one recently seen on the MacBook Air range, where they got a minor processor bump and a $100 price reduction across the range.
Kuo tends to be a reliable source in terms of what Apple has planned, though his record on timings is variable. All the same, if you were planning to buy a new Mac in the next few days, it would worth holding off until WWDC to find out.
WWDC would also be a great time for Apple to release a 4K Thunderbolt display and a 4K iMac to go with it.