Harmattan. Not yet, obviously, but when it's out I see no reason that it shouldn't be supported on the N900 in the same way that the latest iPhone OS releases are available on all the existing iPhone hardware.

Partly because, like Milhouse, I don't fancy buying new hardware just for the bug fixes, and partly because of the effect on 3rd party development. If each release is restricted to one single device it's harder to build up a critical mass of potential users for would-be developers to target. Apple don't go to the trouble of handing out free updates for older devices because they're nice people, they do it because it helps both developers and users feel secure about the future. Nokia doesn't do it, and the result is that people don't feel secure, hence this thread being created in the first place.