After updating the Android SDK installation, my Android Jenkins projects failed with the message "Emulator did not appear to start; giving up".
As it turned out, Android SDK tools r12 now contains two emulators, "emulator-arm.exe" and "emulator-x86.exe". The old "emulator.exe" just starts the real emulator process and then quits. So, when the android-emulator plugin checks the process, it is already dead.

As a workaround, I renamed emulator-arm.exe to emulator.exe, but of course it would be nice to have the emulator platform configurable in the plugin