As vivainio says, Python is also a scripting language, so you can indeed develop on-device.

Python also has the added advantage of being mainstream (lots of examples on the 'net) and actively developed, and also PySide (which provides calls to interface between the Python scripting language and the Qt UI library) is supported by Nokia so has access to most (pretty much afaiu, work in progress) of the widgets and functionality that you can use from C++/Qt.

PyQt is another interface like PySide, and the two have almost identical class/object naming (so you can use PyQt examples without too many troubles), but I recommend PySide as it's the official Nokia version (and Nokia own Qt).