Apple is telling their direct-sales channels (Apple stores) that they won't be honoring orders for iMacs and Mac Minis in the short term, meaning that an update is pretty much guaranteed soon. Unless someone's playing an awesome joke.
These might be those thinner, Blu-ray powered iMacs we heard about last week. The other part of the rumor says that the machines might have an LED display, a unibody build or something about audio. [Apple Insider]