Uh oh! It looks like there's shipping delays and availability issues with 27 inch iMacs. It's reported that these delays could be due to graphics issues with the GPU. Several Apple resellers are citing shipping delays when trying to order the models.
Apple made a brief statement to CNET tonight acknowledging the delays, but iMacs are showing a two week shipping delay on Apple's online store, which might mean you won't be able to purchase one in time for the holidays.
Graphics issues are nothing new to these iMac models. Some 27 inch iMacs also had issues earlier with graphic performance and Flash, which was fixed in the 10.6.2 update. The 27 inch iMac has also had a number of other reported problems before, from machines that have had broken screens, to iMacs arriving that simply don't work.