The flash-and-reboot script basically uses fiasco-flasher to update the kernel and/or initfs partition (that one's no longer present on the N900 anyway) and then sends a reboot request to mce via dbus.

Based on http://repository.maemo.org/pool/fre...03+0m5.diff.gz, the new equivalent might be /sbin/fiasco-image-update and that gets invoked by the kernel-flasher post-install script so an apt reinstall may be enough after all (but don't take my word for it).