For some reason I thought you couldn't do this - but thanks to Dieter (below) pointing out that it's just "AF tracking" that doesn't work during video.
Anyway, I'll leave this video up here to show what touch-af does during a video. It's a really useful feature for quick shots where you want to lock focus on something and keep it there. Obviously if you have more time to set up, you'd do it manually, but nice to have this option.
You have to switch off Continuous-AF in the second page of the motion picture menu, then you can use the various touch-screen modes to focus while you shoot. The lens will stay focussed at that distance until you select something else on screen.
Here, I was using the area mode and touching between the wall outside and the kitchen utensils. The camera focusses very accurately. This looks incredibly sharp in the original - so do download the file and have a look. The Vimeo HD mode sometimes looks terrible (sorry Vimeo)!