Like Thesandlord said it's running on jalimo in combination with SWT for the graphical Interface.

Properly it would be much easier to calculate the speed from the given gps data but we wanted the whole thing to be really geek like and there are some more Physical aspects on this side, perfect to make our Physics teacher satisfied so we decided to use the NXT.
We're thinking about continuing the development on the project and releasing a version where you can choose between low geek mode which is using the GPS for all calculations and is thereby independent from any hardware and the High geek modus where you can connect your Tablet to an AVR Board or an LEGO NXT to calculate the speed from the data given by the dynamo.

Talking to the NXT does honestly look much more spectacular than it really is. There is a project called lejos which is providing an API called Icommand to communicate with an NXT from your computer via Bluetooth. You can use it in the same way on your tablet, you only need to recompile the runtime libraries for ARM. You can also use the standard Linux Bluez API but in this case it's also necessary to compile some libraries.

toti