Users need to have the previous firmware update, Cyan, installed in order to install Denim. (And yes, as a Verizon user, I know Verizon has still not pushed Cyan or Windows Phone 8.1 to us. The latest rumors suggest Verizon is now planning to provide Denim, along with Windows Phone 8.1, 8.1 Update and Cyan, some century soon.)
Update (December 19): Microsoft's Joe Belfiore tweeted on December 19 that Icon users should be getting Denim (plus the 8.1, Update and Cyan rollup of updates) in "early 2015."
Denim includes a number of imaging/camera features, including camera quality and speed improvements; a new "Moment Capture" feature for 4K video recording on some of the higher-end Lumia phones; and support for new imaging algorithms, providing for crisper photos in daylight or low light. These imaging/camera enhancements are specifically for Lumia 830, Lumia 930, Lumia Icon, and Lumia 1520 models.
Before anyone asks, I do not know when Microsoft is planning to roll out a second General Distribution Release (GDR) update for Windows Phone 8.1. There were rumors earlier this year this would happen in the fall of 2014. I haven't heard a new target date.
On the mid- to low-end phone front, Microsoft has expanded carrier billing support to several new markets, including China (through China Mobile); India (with Idea); and Brazil (via Claro); and the U.S. (via Verizon Wireless). Carrier billing allows users to purchase apps through the Windows Phone Store without a credit card and instead via their wireless providers.
Following today's announcement, Microsoft now has carrier billing arrangements with 81 carriers in 46 markets, which officials say represents 46 percent of all subscribers worldwide, based on Ovum World Cellular Information Service data.
Earlier this week, Microsoft also made available an updated version of its free Windows App Studio tool, which enables app creation in a simpler way. The updated version adds support for the TouchDevelop touch-centric programming language; support for universal Web App Template (WAT) creation; and compatibility with the recently announced free Visual Studio 2013 Community Edition product. The new version of Windows App Studio is phasing out support for Windows Phone 8.0, however.