OK, so I finally had the time to try this (sorry, I had almost forgotten about it...); and, thus, I did a:
Originally Posted by rsavage
... and modified the two append sections with:
gksudo gedit /etc/yaboot.conf
... thus adding the fan adjustment entries; finally, I did a:
append="quiet splash therm_adt746x.fan_speed=38 therm_adt746x.limit_adjust=5"
... and rebooted.
Well, now the iBook G4's fan indeed seems to activate later and is also quieter, which is especially noticeable when running from the mains (before, in that condition, it was quite loud from the beginning).