Apple has today started allowing customers to place order for the brand new 2012 iMac, both in Apple retail stores and online through the Apple online store. The first batches of 21.5 inch iMac models have apparently reached Apple retail stores and will be available for walk in purchases, although the stock will be pretty limited in the first instance. Customers can also place orders for both the 21.5 inch model, with a base price of $1299, and the 27 inch model, with a base price of $1799 online.
The 27 inch model will not start shipping until December and is not yet available in retail stores, as Apple has stated that it will not launch until December. Currently, the 21.5 inch models are still showing as shipping in 1-3 days, so have not sold out yet. The 27 inch model is being shown as shipping in 2-3 weeks, meaning that it should launch in the first half of December.
As Tim Cook has previously said, it is likely that stock will be limited in the first instance, so if you are planning on purchasing a 21.5 inch model, best to get your order in quickly.