I think we should start with a native Debian squeeze port to the N900, while legally reusing the proprietary Nokia components to avoid losing functionality (unlike Mer).
This means we would try to upgrade as much as possible free components to their Debian counterparts without breaking compatibility for the proprietary Nokia stuff. Alternatively the Nokia software could run in a minimal chroot to provide everyday functionality on the N900.

I think starting with an existing device has the advantage of testing the port in the real world scenario
and access to a larger community which may be interested in running a full Debian distribution on their device (so far they only have Easydeb).

Later we could integrate more and more MeeGo components and send the necessary modification to
Debian upstream. Building of packages would be done using the Debian infrastructure.

I'm working on a solution which makes it possible to keep the original rootfs on NAND
but which creates a chroot on eMMC and boots from there. It removes the need for optification
while taking advantage of the faster NAND for system files. System files can be selectively replaced.