Do you have the source code for the N900 camera app? Can you add buttons to it? What about the source code for the phone app? Can you add buttons to that? You can for the galaxy S, for example, Power button was hacked to be used as camera Shutter button. About drivers I don't know. About Nexus S, there were reports to both ends, some saying they don't know yet, some saying yes. Personally I think it won't, cause that will make the Galaxy S last longer, preventing users from buying new phones.