There has been some talk how games will be controlled without a d-pad. Here is a good example. It is Metal Gear Solid on the Iphone. This is assuming that the N900 has a Capacitive touch screen that supports Multitouch. Which from reading this thread and it not having a D-pad suggests to me. You can skip to 2min 40 sec.
http://www.youtube.com/watch?v=ae00iK64EkU
They could also use a move able on-screen D-pad

Here is a trailer for one of the games included on the N900
"Bounce" it is controlled with the accelerometer
http://www.youtube.com/watch?v=s81JUXd2RCc

Here is my song
"I'm so excited and I just can't hide it"
Pointer Sisters