Update [fixed]: I got it to appear by opting into the beta of SteamVR itself.
I put together this guide for those who want to get everything set up right from scratch: http://www.roadtovr.com/how-to-enable-oculus-rift-dk2-half-life-2-steamvr-steam-big-picture-virtual-reality-mode/
On a somewhat old but functional Windows PC (can run some Oculus Rift DK2 stuff, but not much). Just installed a fresh copy of steam today after reading about the DK2 update.
I've installed the SteamVR tool and the Steamworks SDK tool, restarted my computer, opted into the Steam client beta, and made sure my DK2 is set in extended mode. Still, I don't see the VR Mode option under Steam's View menu.
I've also installed Half-Life 2, and I can see the VR mode option in the settings, but it is grayed out.
I have the Oculus Rift 0.4.1 runtime installed, the firmware updated, and it works fine when launching the Oculus Desk demo scene from the configuration tool.
Any thoughts? Is it possible that my old PC (mid-range gaming desktop from four years ago, running Radeon HD4600) is lacking something needed to even see the option in the menu?