fourier transform of smart-phone accelerometer in matlab
Two methods occur to me.


Perform a non-uniform DFT. You can use Google to find MATLAB implementations of it, or try to write one on your own. This file seems to be legit, although I haven't tried it myself.
If you have the Signal Processing Toolbox, then you can use the resample() function to interpolate your non-uniformly sampled vector linearly onto a vector of uniformly spaced instants, and then perform the usual DFT on the latter.