This Pycon 2013 talk dives into Kivy, an open source Python library for building cross-platform GUI and mobile apps:
Kivy is running on Linux, Windows, MacOSX, Android and IOS. You can run the same code on all supported platforms.
It can use natively most inputs protocols and devices like WM_Touch, WM_Pen, Mac OS X Trackpad and Magic Mouse, Mtdev, Linux Kernel HID, TUIO. A multi-touch mouse simulator is included.
(There's also a
this month to add Raspberry Pi to Kivy's already-impressive list of supported platforms. Check it out if you're interested!)