(* Solved, read the next post. *)

I confirm what petzik said. Only on Maemo 4 the "python2.5" package does not contain all of the Python 2.5.2 files (on Maemo 5, "python2.5" has the usual Python 2.5.4 files).

So SymPy on Maemo 4 needs another dependency, "python2.5-dev" which contains the missing files "doctest.py", "unittest.py" and "timeit.py" in "/usr/lib/python2.5/". I have had "python2.5-dev" installed on my Nokia N810 so I overlooked this problem. Sorry for my fault.

So the (temporary) solution is to install the "python2.5-dev" package (14.8 MB after install) :
- open the X-terminal;
- become root by typing "sudo gainroot";
- type "apt-get install python2.5-dev" and confirm.

One problem on Maemo 4 is that "Integral"/"Derivative" will install python-sympy, python2.5-dev and ttf-dejavu, about 40 MB. For N8x0 users without cloned Maemo in internal SD, it can be an issue to have this free space.

(* Released *)
I will soon release a new version of "python-sympy" for Maemo 4 including "python2.5-dev" as a dependency.

If somebody has a better and easy to use solution for this problem, feel free to share here.