toyota did release a firmware update for the 04 and early 05 priuses, and one previous to that for the early 04s (i think) the newer one is SSC 50P and the older one is SSC 40D, both recall issues.
they require a toyota scantool to load the software, and the software is loaded onto the scantool from toyota's internal network.
it's no child's play. takes quite a while for the new version to load, they actually have to use a battery tender for the 12v battery.
not all ECUs are updatable. the NAV system can be updated with a CD, but i don't know of any other software updates for the prius.