Hi again,
and thanks again. Unfortunately I didn't explain me very well...
I had a good MMC booting setup: I mean, I was able to launch maemo both from internal flash and from MMC.
The problems came when I tried to install KDE: after that, starting from MMC I wasn't able to do nothing at all!
This is why I reflashed my N800, reinstalled everything and tried the .deb way.
As wrote before, the only idea I have is or that the new OS is not compatible with KDE, or that my 2Gb MMC is somehow too small for the .deb tarballs.

Anyway, thanks again. All the best

C.