First, it sounds like he'll still be working for HP Enterprise as his "day job" while working for Valve. Then in terms of his technical focus at Valve, he wrote on his blog:
First thing on my plate is helping support head-mounted displays better by getting the window system out of the way. I spent some time talking with Dave Airlie and Eric Anholt about how this might work and have started on the kernel side of that. A brief synopsis is that we'll split off some of the output resources from the window system and hand them to the HMD compositor to perform mode setting and page flips.
After that, I'll be working out how to improve frame timing reporting back to games from a composited desktop under X. Right now, a game running on X with a compositing manager can't tell when each frame was shown, nor accurately predict when a new frame will be shown. This makes smooth animation rather difficult.
Brief and to the point.