This has been discussed at length elsewhere, but Nokia is generally supportive of seeing Maemo 5 on current hardware, and the community interest seems to be there; I personally am quite confident it'll happen, but there are some doubters out there.

As far as Diablo, I don't think so. I wouldn't go as far as to say integrating proper A2DP support in Diablo is as big a project as the Fremantle backport, but it's close enough to be a poor option; resources are better put toward the Mer/backport effort, where much greater returns can be realized.