In case it is not 100% clear, you are free to develop on any platform; iPhone, Android, Symbian, Maemo, OpenMoko, Qt, Palm Pre, Mobile Python, Java etc.
As was mentioned here, we have allocated space at the venue for hacking and will be providing power and internet connectivity, with the rather reasonable limitation being that you should be hacking on mobile technologies.
Check the other posts on this site for more links and information.
The Judges will be looking at criteria such as;
- coolness/wow factor of the application
- how much of the code was created/written during the hackathon
- which platform(s) the app is aimed at
- does the app crash during a demo in a spectacular way ;)