I really like D(2) language and would like to use it for multi-platform GUI application, but I see that its ecosystem is not quite developed. After moving from Linux to (Free)PC-BSD, I see there is even no 64bit compiler ready in the ports and one is not sure if any of GUI libs (QtD, gtkD, wxD) are ready for serious project and we would like to start with the project as soon as possible.
Otoh, I was previously recommended to try with Python which is mature, with many tools, GUI libs etc. but there was question about speed - we have to use one C lib for calculating Ephemeris and write several libraries which would use that C-lib. 
However, this might be good (perfect) job for Cython, so my question is what do you think about writing GUI-part in Python (Qt,EFL) and use Cython for performance-critical stuff (binding external C lib and writing our own libraries) instead of (waiting for) D to become ready for serious projects?
Writing the GUI in a dynamic language and writing performance-sensitive code in another language is a great way to write an application. This can be done in Python, it can also be done really easily in Tcl/Tk and several other scripting languages.
multi-platform gui app in C(P)ython or D