A look at the PyPy 2.0 release

Sorry, I don't have direct experience with SIP, but perhaps your GIL question is answered here:

http://pyqt.sourceforge.net/Docs/sip4/using.html#the-pyth...

Well, that's good, but I'm still missing other features I get with Boost.Python[1] in minimal code (typically ~3 lines of C++ per typical method (with named args and a docstring); add ~10 extra for more involved methods including virtual methods, callback wrapping, and PyObject-as-std::iostream translations). Other features are custom getter/setter functions for properties (boost::variant bindings with named accessors which return None if the value for name isn't set and can be assigned to), protected class access[2] (we allow inheriting from a a C++ class in Python and calling it from C++ again), and the extract class mentioned above (boost::any bindings). Not sure how well SIP handles using objects via a shared_ptr (or similar class) and never by instance either, but this is likely hackable with some work.

[1]With which my main gripe about is the lack of PyPy support. This isn't me just ripping on other bindings systems for the sake of it; these features are just required for complete bindings and that is more important than PyPy support (as nice as it would be).
[2]"SIP does not support access to either private or protected instance variables."