In simple terms, you can think of a .deb file as a .zip file (actually it's a .tar, but you may have come across .zip more often) - i.e. it's an archive containing the software and supporting data files. Just because you can get an iPhone app in a .deb, doesn't mean it will work on an N900. The same as a zipped up Mac OS app won't work on Windows and vice versa. Hope this helps.