How do you know when major corporations are beginning to consider open-source CMS packages? When major analyst firms start writing about them; this means they're getting questions from their corporate customer base. Witness this draft analysis by Gartner (thanks to Pascal Van Hecke for the ref.). It's typical Gartner fare: very intelligent analysis of the broad landscape combined with highly inaccurate evaluations of specific packages. However, the analyst does win points for soliciting the opinions of real developers. But more importantly, the fact that Gartner (and Giga and Jupiter) are examining open-source solutions for content management should tell you something's afoot out there...
Check out the Gartner Draft