The Steam client beta has been updated as of yesterday, introducing a variety of changes, among which is the inclusion of DualShock 4 support, something which Valve said was coming in the future during Dev Days in October.
Within the 'Controller' section of the patch notes, there is a detailed description of how the DualShock 4 support is being introduced:
"Added Steam Configurator support for PS4 Dual Shock Controller. Note that this is a beta release. Enable in Big Picture settings Add/Test Controller settings.
When enabled, PS4 controllers will have access to the same sort of customisation/configuration support as Steam Controllers, including native API support. PS4 Controllers using this system can map the trackpad, gyro, buttons, etc. to keyboard, mouse, or x-input outputs and can make use of action sets, touch menus, radial menus, and so forth.
Added New Mode - Joystick Mouse. This is a joystick based mouse control useful for stick based controllers.
Added default PS4 templates.
Added software calibration for non-Steam controller based IMUs to counter for gyro drift.
Configuration Browser can now be toggled to show all available controller types rather than the default of the in-use controller type.
Configurations can now be marked as "Unique" to a specific controller. This configuration will only be applied to that controller for that user's account. By default, all controllers of a specific type will share a configuration unless marked as Unique.
Big Picture now supports PS4 controller glyphs when using Steam Configurator support."
Of course, this is the biggest news coming out of the update, however, other changes have been made as well, including a fix to third-party mods showing up as the incorrect game name in friends lists and improvement of the error message that appears when attempting to run a 64-bit game on a 32-bit OS.
The full patch notes can be seen here. Is the introduction of the DualShock 4 support a big deal for you?