I can't imagine this is not possible.... but I can't figure out where to enable it.
can't system monitor (
gnome-system-monitor) display disk io?
Why you don't use iostat :
iostat is found in sysstat package .
For example :
will give you I/O result in 30 s interval
I gave up on gnome-system-monitor for this reason.
On my 12.04 machines I installed indicator-multiload. Once installed you can use the Preferences page to get it so show disk i/o - along with memory, swap, network, etc...
You can use system monitor from KDE (ksysguard), open it, go to file and click on "download new tabs" then, you can download disc io.
Anyway, lots of tools let you watch disc IO, for example gnome-shell plugins, Unity indicators, KDE plasmoids or conky.