well I can't say about the usage of open source may be it is useful but usually while developing ur own application, it is better not to use open sources.... I am doing research in Nokia's S60, and planning to develop Novel User Interface, that is creating a 3D world and some characters, the main theme is to play ID3 songs in the cell phone, and the 3D world will be according to the genre of the music that user choose... these are the basic concepts and as per the company's policy, I am not authorized to give all the details... and the software we use is carbide C++, active perl, java somewhat....