As I do understand this, Mer is Maemo5 ported to n800/n810. this port is supported by Nokia, however most of the work is done by community, not Nokia.
So it depends on volunteers, how this project moves forward.

As Maemo5 is still not complete, Mer is moving target. Moreover, lots depends on how can Maemo5 be supported on n800 hardware. (and probably on availability of promised 3d support as well.)