Hello, I am developing Mac softwares on PowerBook G4 12 inches with, mainly, Mac OS X 10.4.9.
Some of the softwares of mine are compatible with Leopard and make the best use of Leopard functionality.
I hope I can upgrade my Mac to an Intel based Mac some time in the near future, and also broadband internet, as well as iPhone developer kit (of course iPhone or iPod touch too!)