The problem is that under rescue mode everything is read only,
Now that would make for a pretty worthless rescue mode!
I suspect that your problem is in using /etc/fstab instead of just etc/fstab.
On my FC4 rescue cd, I type chroot /mnt/sysimage
Then, I verify that I'm looking at the hard drive fstab and not the one on the cdrom...
press the insert key
press the escape key
then check your work again with cat etc/fstab