One of the major reasons Fremantle won't run on Maemo 4 devices is the lack of OpenGL support on those devices, as Fremantle makes use of the clutter OpenGL scene graph library for its user interface. While the N800 and N810 do have OpenGL hardware, the available drivers were never integrated into the OS, and as far as I know, they aren't any good anyway, which is probably why they weren't integrated.

With running Harmatten on the Fremantle device there hopefully won't be any similar problem of the older device lacking a major required capability, at least I can't think of any today.

In fact, if you look at Quim's slides, you'll notice that clutter is being dropped for Harmattan as well. Presumably what is being done with clutter in Fremantle will be done with QGraphicsView and Qt's new declarative UI technology in Fremantle. Qt's graphics stuff has multiple backends, such as software rendering, XRender, GDI and OpenGL, as well as a couple of others relevant to other platforms it supports.

So, the Harmattan desktop, being built using Qt, would presumably still make use of OpenGL acceleration -- but it might not actually require OpenGL support, unlike the Fremantle desktop. This might make it far easier for the Mer people to backport it even to the N800/N810 (but the performance of a desktop targetting a much newer and faster device might not be acceptable on them -- still, I think the switch to Qt is quite possibly great news for N800/N810 owners who don't see themselves buying a Fremantle or Harmattan device).