I have the same problem with both Xperia active and Xperia arc: Afte update to Android 4.0 some virtual sensors are gone, namely gravity sensor and rotation vector sensor.
Screen rotating and most other orientation-based features are not affected but some specific apps that use the rotation vector sensor are.
Below is a screenshot from the Sensor-List app on my Xperia active, which shows the available sensors. Before the firmware update the rotation vector sensor was definitely available for I've used it myself in an app I developed. Unfortunately my app won't work anymore since the update...
From a developer's point of view using the rotation vector sensor is much better than using accelerometer and magnetic field sensor directly because with the latter much more calculations and filtering is needed.
Edit 13/02/11: Well, a nice DIY solution for Android developers is to implement their own rotation vector sensor, as described here.
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