While researching why some students' activities were not showing up in the logs/reports, I discovered that I could reproduce this by using the Moodle Mobile app on an iPhone. I downloaded the latest version for iPhone 4, as far as I know. I logged in, poked around in the course and checked the activity logs on another system as I poked around. None of my new activity showed up.
I just installed version 1.9 on my Android phone and experienced the same thing.