To reliably detect all possible rotation with only linear acceleration sensors would actually require three sensors that are not placed on a same line.

With only two, you could position the device so that the axis passing through both sensors is aligned with gravity, and then rotate around that axis to make the device lose it's orientation.

I had created an app called pyaxelwii for logging wii accelerometer data for the N800/N810, I couldnt get meaningful results from the logged data. From the posts above, I think it may be due to not storing the accurate time stamps between the data.
there is an app called accdisplay for N900.
how about using the wii motion plus remote along with the n900 accelerometer to do the calculation.

There are two very basic problems:

1) You need acceleration in all degrees of freedom (translation x/y/z and rotation x/y/z) Wiki link. The accelerometer in the n900 only measures translation, not rotation

2) You'll have to integrate twice. Integrating acceleration gives speed, integrating speed gives position. You'd also sum up errors. Unless your accelerometers are very very precise, you position will drift very soon. The accelerometer in the n900 is pretty inaccurate

If by "accelerometers" you mean the device that has 3-axis, like the one on N900, then you need two, as long as they are not aligned, as I said to navigate (I'll expand on that).

Your suggestion doesn't work because it's basically impossible to align and rotate a phone like that in real life. Also, in real life, rotation will show up on accelerometers because of centrifugal force.

You're right, in theory, it is possible to do that. In practice, I very much doubt it. For one, if the angle isn't 45 degrees (and it isn't), rotation will induce centrifugal on axis of each accelerometer at different rates because they have different angles to the axis, so the resultant will show different accelerations as left and right on each.

It will show up as rotation on the wrong axis. So, IMO, not worth it for a phone. Definitely a must for inertial navigation. Also, planes use gyros, not inducted weights. Also, they cost 900 N900s.

So, to summarize, you are correct, you need 3 to accurately detect all movement. But by the time that influences your navigation (it doesn't because navigation doesn't care you roll when navigating), you're hit by several other issues, related to precision, drift, sampling rate, etc.