Q: accuracy of android "linear acceleration" versus manual processing of accelerometer? Reading here: Android Sensors - Which of them get direct input?  ,
I am wondering if anyone has experience or a technically detailed link about the accuracy of the linear acceleration versus just manual processing of the acceleration raw data. E.g., do the new phones have dedicated hardware chips for filtering out gravity, or are most devices just going to filter the same raw source? 
Update, proposed answer for someone to confirm if they have such a device (Xoom,Nexus S,?):
"If the device has gyro, or possibly multiple accelerometers, then the returned values for gravity (G) versus external linear acceleration (L) can be fundamentally more accurate than any processing on accelerometer data alone. Without extra sensors, e.g. as on most phones, one could in principle post-process the Acceleration (A) to attempt separation as accurately as what the device is returning for A = G+L"

A: It seems, the gravity/lin. acceleration can be calculated by a low-pass-filter - just as described in the Android-Documentation.
However only filtering the last value will not do it. I get acceptable results by averaging the accelerometer values of ca. 200ms (for moderate movement, this will still screw up, e.g. when you flip your phone fast between your fingers).
Your proposed answer is most likely correct.
You can check the statistics of several smartphone models on Android fragmentation.
For many models the power consumption of the lin. acceleration and gravity sensor is just the sum of accelerometer, gyroscope and magnetometer. 
The gyroscope lets you recognize fast angular movement and it can be used to improve the gravity value, which is not possible with just the low-pass-filter. For the magnetometer im not sure if it really gets you more information.
On my phone (HTC One S) the gravity sensor uses just as much power as the accelerometer, but is still better than my simple filter. So either it is another hardware sensor or probably they use different weights on it. I tried to weight acc-data stronger, if their absolute value is closer to gravity, which is nice but was still not as good as the actual gravity sensor.
For compatibility reasons I would suggest to use a low-pass-filter for gravity if possible, as still not every smartphone has a gyroscope or mentioned sensors.
