The only situation where anything developed on x86 would not also work on ARM is if you include native code via the NDK (which is not usually done except where you need to wring every last bit of performance out of the device), and that's just a matter of building the native code for all the platforms you intend the app to run on.
The platform used by the emulator image makes no difference to any of that, it just makes it a lot more practical to get usable speed out of it.
I think that's overstating it a bit. The NDK is very heavily used by the overwhelming majority of premium games, not least because it allows for significant code reuse between Android and iOS. The release of the NDK was the beginning of a massive quality app explosion on the platform.
e.g. (in the Application.mk)
APP_ABI := x86 armeabi armeabi-v7a
This app will run with gust on any of those platforms.
"Last week’s r17 developer tools release included x86 system images and host drivers (available through the SDK Manager), allowing the emulator to access the host CPU natively and offer significantly faster execution."
I just wonder, does this mean that the apps built on this version will only work on x86 devices and you need to modify the app later to work on ARM devices, too? Because if that's the case I probably won't bother with this, since 99.9% of Android units have an ARM chip.