Hm... In Diablo the kernel-diablo-flasher package doesn't actually flash the kernel to the mtd. That part of the process is initiated by the application manager (triggered by a "Maemo-Flags: flash-and-reboot" header in the package) so it doesn't happen when (re)installing via apt. One has to run /usr/bin/flash-and-reboot manually to do it in this case.

I'm not sure if Fremantle is similar, but if so someone should update the wiki with more complete instructions.