I'd be excited to have some computational photography and computer vision on the N900.

here are some more ideas:

* photo-watchdog:
many industrial cameras can be configured to read out
a subset of their sensor with much higher framerate.
If that's possible on the N900 camera,
an application could watch a selected region (e.g. finish line)
for a change with very high fps and take a picture or start
video recording as soon as something changes.

* Camera-shake image deblurring:
might be to computationally demanding for the CPU
but the EXIF could at least contain all acceleration sensor data for later postprocessing

* more intelligent Autofocus modi (like professional DSLRs)
with software AF points

* online Face, cow and car detection (CVs love that! ;-)

* fast object localisation: specify some target images (e.g. one sock) and get alerted when and where something matches,
see e.g. http://www.kyb.mpg.de/publications/a...MI_[0].pdf

* live-preview as desktop background?

for the user experience: if some algorithm is too slow, it should subsample the video stream and indicate the fps in the preview.