my upgrade from Panther to Tiger appears to have replaced my /etc/hosts with a default one, without backing-up the original. But I've got a back-up somewhere else. So should you.
Update: On some other machines, Tiger appears to back it up to /etc/hosts~, but I think i already had an older copy from a past upgrade at this location, so my /etc/hosts~ is the older older file, not the one I wanted.
In any case, if you care about your /etc/hosts, just back it up :)