If performance is not a problem, why not use only python without obj-c? wxPython provides a natural look for both Windows, Mac OS, and Linux. And it's easy to make distributions for different operating systems using py2exe and py2app (and linux users still have python :-) .
BTW: There is a great example of the open source wxPython application - Editra text editor. In addition, the author of this program wrote a good book for developing cross-platform applications using wxPython. , . - 50%:-), .