Input.compass.rawVector returns zero vector when it's called for the first time. This is reproducible on Neuxs 4 with Unity 4.0.0f7, 4.1.4f1, 4.1.5f1, 4.2.0f4 and on Galaxy Nexus only with Unity 4.2.0f4.

Tested devices:
Google Nexus 4 with Android 4.2.2
Google Galaxy Nexus with Android 4.2.2

Unity Versions:
4.1.5f1 (ed5ea3281df6)
4.2.0f4 (38efbd14869d)

Resolution:
The compass is being enabled in the script, which is called after the input data has already been polled for the frame, so it returns default zeroes for the first frame. In the next frame the compass is polled in time, and the data is available.