It appears that Apple has discontinued its 20 inch Cinema Display and according to Mac Rumors the product has been given the EOL (end of life) tag. It is easy to assume that Apple is getting ready to replace the older monitor with a new LED based display to keep it inline with their other products.
Apple's business model is to phase out one product before the introduction of an upgraded model. While similar to other business strategies Apple sticks to this model for all of its products and does not waiver in the slightest bit.
As previously reported on Neowin, it is expected that the Mac mini will receive an update in the near future so it is not to hard to believe that the monitor will receive an update around the same time. It is also possible that Apple could remove the 20 inch model completely from its offerings too.