There has been an ongoing discussion/debate about whether accelerometers are actually useful for attitude estimation on flying rotorcraft (quadrotors, in particular). The simple, one sentence answer is "yes, they are."
However, the accelerometer measurements are non-intuitive. If the estimation algorithm uses the accelerometers incorrectly (which is, incidentally, the norm on hobby and even some professional aircraft) then there will be significant errors.
There is some good literature available now that describes the problem. Martin and Salaun published a very good paper describing quadrotor flight dynamics. I also just finished my MS thesis, wherein I address the subject in detail.
In my thesis (the last link), Chapter 5 is what you'll want to look at.
Thanks for your helpful post Caleb. Would you possibly know an answer to the following question:
Attitude estimation with ONLY 3axis ACCELEROMETERS. and nothing else, based on a lowpass filter? where can I find an opensource way to do this?
Why do you want to use just accelerometers? You can add gyros without increasing your footprint size - Invensense sells a chip that combines gyros/accels into one small package and a common communication interface.
You COULD use only accels, but your results will be very poor unless the system that it is mounted to never moves or only moves very very slowly.