Hi Romain,
    I just wanted to chime in that I think a Python backend for Cython would be awesome. You wouldn't want to generate bytecode for compatibility reasons: PyPy doesn't use exactly CPython's bytecode, and afaik IronPython and Jython both just use their host VM bytecode directly. Generating ctypes+pure python sources would be the most generally useful, and I think that'd be the best GSoC project.
    Some great follow-up work would be to skip ctypes on PyPy and generate different calls, as Antonio has found that even with his awesome optimizations, ctypes is significantly (I think it was 5x) slower than the alternative, rawffi I think, but don't quote me on that or the number.
    As for reviving the jvm backend, I think that's a very *interesting* project, I actually have looked at doing that myself, there's a tiny bit of work in the ootype-virtualrefs that I've done. You'll need to learn a ton about how RPython works, the translator, and so on. I'm not sure that one would get approval though, especially pitted against awesome projects like your ctypes project. But don't let me discourage you from applying for *both*, I just wouldn't put all my eggs in the jvm-backend basket.

Cheers,
Dan