If you hit the button in settings->storage you're actually running a linux umount command. What will need done after that would be to run the mount -o -rw,remount -t command on the sd card. I know taking out the card and re-inserting it works, unless you've flucked up the automount script (I'm sure you haven't), and rebooting the phone calls automount.
On the Hero I have here in my hands, the only way I can see to remount the card AFTER you've manually unmounted is to open a term and remount it, or remove/reinsert, or reboot. Looks like the option to unmount it is only when you want to remove without powering down.
If you mount/unmount from the notification bar (as when you plug in to your pc) it runs a more friendly script that gives you the option to unmount from device and mount to the computer, and unmount from computer then remount on device.
Short version - unless you're going to remove the sd card, don't use that button