First, it's interesting that Mixpanel and Optimizely (both YC companies) are starting to compete in this space. Optimizely has mobile A/B testing as well[0].
Second, what seems to be an obvious need I haven't seen is A/B testing notifications. Notifications seem to be the "email drip campaigns" of the mobile world, so it seems like A/B testing both the frequency and content of those could bring big wins.