The spec for the install/update of grub.conf does that. In FC3 the new kernel stanza is written as the first entry and pushes the others down. The default is not changed from 0 so it automatically selects the new one to boot.
I believe that FC2 also wrote the new one as the first entry in
grub.conf, but it updated the default to point to the old entry and you
had to manually select the new kernel until you updated grub.conf to
have the new one as the default.
In FC3 this behaviour is controlled by the setting of the variables UPGRADEDEFAULT and DEFAULTKERNEL in /etc/sysconfig/kernel, as mentioned in the release notes.
Paul.
