There's no doubt about it, people want to use Bitcoin on their mobile devices and, in such a connected world, there's no particular reason that mobile woudn't be the next frontier for Bitcoin. But mobile security is a pretty scary place.  So my question is pretty simple:

In your opinion, what is the safest way to run a Bitcoin wallet on a mobile device? I don't mean a watch-only wallet, I mean a fully functional wallet.  Let's assume for this discussion that "don't" and "convince the OS makers to make security better" aren't valid answers since those don't address the need right now.