All right...
Using ;debugOn and ;dumpMessages ; I looked at what was going on:
091024:105149 EXT3 FS on mmcblk0p1, internal journal
091024:105150 system: I _otaupexec:def

rocessing update /mnt/us/update_test-k2i.bin
091024:105150 system: I _otaupexec:def:version is "FC02"
091024:105150 system: I _otaupexec:def:update image checksum OK
091024:105150 system: E _otaupexec:def:signature does not exist for "install.sh"
091024:105150 system: E _otaupexec:def:signature verification failed
So either by signature it means the signature found in the .dat file and the calculation has changed ; or packages are actually signed from now on , killing user updated...