Today I found several Mathematica packages for doing things in general relativity and quantum field theory, I updated the links here. I found xTensor, SusyMath and cadabra (this one is written in C++). Many more can be found in the Computer Physics Communications journal (CPC). Those packages are opensource, but depend on a proprietary product. And I cannot help myself, but when I look at the sources, I just find the Mathematica language completely messy.
So now I am concentrating to get the relativity.py working and then calculate something that they did. And if it turns out that it is reasonably fast (i.e. I will get the correct result in minutes, or hours) and if the source code is going to be short and nice (I hope so), I'll try to write an article about it to CPC. As a side effect, it will be a real world test for SymPy. Also I would like then to do the same calculation in SAGE, to see how it compares to SymPy. What I don't like in SAGE is that I prefer to import the library as a normal python module and I don't want to be trapped in another environment (SAGE is actually preproccessing the input a little, like converting ^ to ** etc, which I don't like at all). I think the symbolic package should be a regular standard python library. And who knows python, should easily understand how to work with that.
Right now I am struggling with the mattern matching (issue 82) in order for the dsolve() to be able to solve the differential equations that arise in relativity.py.
I would like to make it into 0.4. I think we can release a little after sympy gets into debian unstable (hopefuly in a week or two), so that we can check how it behaves etc. and fix everything for 0.4.
I know that there are still some issues, that are quite embarrassing they still don't work in SymPy, but I think more important is to "release early, release often".