Glyn Moody writes: Two years after its launch, Google's Linux-based Android platform is finally making its presence felt in the world of smartphones. Around 20,000 apps have been written for it: although well behind the iPhone's tally, that's significantly more than just a few months ago. But there's a problem: few of these Android apps are free software. Instead, we seem to be witnessing the birth of a new hybrid stack: open source underneath, and proprietary on top. If, as many believe, mobile phones will become the main computing platform for most of the world, that could be a big problem for the health of the free software ecosystem. So what, if anything, should the community be doing about it?