A lot of the applications that you guys describe are in fact not considered computational photography. For example, recognizing street signs or the pieces on a chess board are computer vision applications, not computational photography. Computational photography refers to image acquisition methods which combine camera(s) and computation, the output is a photograph (in contrast to computer vision, where a computer tries to understand an image and for example outputs the positions of pieces on a chess board).

I have some neat ideas for computational photography on the N900 but I'm not yet sure if they are feasible (the N900 does not have a lot of computing power when talking about state of the art image processing).

For a computer vision application, it would be cool to have an application that recognizes URLs in images and makes them "clickable". For example, if you have a magazine with a URL in it you can you take a picture and open the URL in the browser. It would be even better to do this for video in real time, not just images.