Perhaps, nobody else has pointed this out but..

How on earth would AR applications take off on the n900 if it doesn't have a digital compass built in? Assuming if you did get a digital compass on the n900 working then you have the problem with you have to assume the developers of such AR applications also have an n900 + digital compass. Or that the next Nokia device comes with a built in digital compass and that the application can be back-ported.