Would gladly test apps when we get to that point.
Right now we need someone who can port the ubuntu app, or if possible, write an app. I don't know how much that will take to write such an app, as it is primarily calculating that needs to be put together, alongside with a shiny GUI.
Actually, the graphics i might be able to help with (but nothing pro ).

As far as the calculation goes, we can find the formulas with a little research. And for using the GPS, local time prayers should not be a problem either - the app choosing the nearest programmed city...
For the direction of the qibla, I don't know if the n900 can do some digitcal compass action, therefore it might be restricted to only telling you: NE or SW or as something like this:
http://upload.wikimedia.org/wikipedi...la_compass.png

I see a mini project here.