With new Android tablets having NVidia dual core processors, clearly the horsepower is now available in Android 3.0. When will Unity support Terrain on Android (without having to convert it to a low res poly)?
It probably won't for several more generations. Even if tablets have multi-core GPU's, the phones are still considerably further behind and make up enough of all Android devices that you would be limiting your potential market right off the back.
Even on GL ES 2.0, terrains aren't feasible on most Android devices or iOS platforms.
So short answer, not anytime soon.