If there really is a backup plan it most likely will be to make Nokia OS (Series 40) compatible with Android applications (atleast the JVM apps anyway).
That is my guess anyway based on the inclusion of touch in the latest version with a UI similar to MeeGo and all the updates to the Series 40 Java SDK.
I believe Nokia isn't allowed to use a different external OS for at least 3 years if the rumours about the microsoft deal are correct, this way it would bypass that problem and still switch to Android (more or less).