ROS is a framework, it's not an application on its own. You can develop on top of it. Nodes are deployed on different computers on a network. One or more nodes could be hosted on N900, nodes that requires lots of processing power could be hosted on other more powerful computers (image processing, storage, ...). Each node provides services ("compute a trajectory", "move an arm"), messages can be exchanged between nodes and they can react ("when hitting a wall, drive back"). This is quite abstract, but it's very powerful. You can follow ROS tutorials to have a better idea of ROS concepts, it's extremely well documented: http://www.ros.org/wiki/ROS/Tutorials

There are several robots which are ROS ready, like Aldebaran Nao, WowWee Rovio, see http://www.ros.org/wiki/Robots. I hope to find time to play with it and N900, and will give some feedback here.

Cheers
Seb