B&H PhotoVideo...
also appears to be facing shortages, six out of thirteen options aren't available. I believe there were only three configurations out of stock a few weeks ago.
Out of Stock: 2.93GHz i7 8GB/1TB/256GB SSD; 2.8GHz i5 8GB/1TB/256GB SSD; 2.93GHz i7 8GB/2TB; 2.93GHz i7 4GB/1TB; 21.5" 3.6GHz i5 8GB/2TB; 21.5" 3.6GHz 4GB/1TB
Many of the models left in stock are fairly standard configurations - no SSDs or upgraded hard drives, and only two 27" models with upgraded (8GB) RAM.
If you go to configure an upgraded processor/HDD/SSD/RAM from MacMall, shipping times increase to 5 to 7 days (from same day) which is another rather good sign of an update within the next few days.