N900 camera sensor
The main camera is a ET8EK8 by Toshiba. The datasheet for this part is not publically availabale.
It is compatible with the N96, N85,N79 and N900. [1]
The service manual for the n900 indicates that the camera can be replaced. In principle this would allow upgrading the camera, however it is perhaps unlikely that a compatible camera of better quality will be easily available. There is also the obvious warranty issues - the camera is not a user servicable part.
The lens cover on the phone can be sensed by the phone, to open the camera application automatically when the user opens it.
The driver presents the camera as a video4linux device - /dev/video0.
The camera is used by several applications, including the built in camera application. It is connected via a multiplexor to the single camera bus on the CPU, along with the front camera. The two cannot be used at once.
This lens is considerably smaller than the minimum physical requirement for a true 5MP camera.