They go here:

# ls -l /etc/ssh/keys-root/authorized_keys
-rw------T    1 root     root          1572 Jul  9 22:24 /etc/ssh/keys-root/authorized_keys

You can put them on the kickstart server and get them during install using this line in the ks.cfg file:

# install ssh keys
wget http://10.101.40.41/preseed/ESXi/5.1/authorized_keys -O /etc/ssh/keys-root/authorized_keys