What you're referring to is a magnetometer. A hardware device that measures magnetic fields and can determine the direction of magnetic north.

As a hardware component, you wont see one added to the N900. But it's likely to be present in the next Meego device as magnetometers have become quite common on high end smart phones since they were first widely included in devices running Google's android operating system.

The iPhone didn't add one to it's 3GS model until over a year after the HTC Dream (T-Mobile G1 in the USA) premiered with a built in magnetometer.