You could probably put your own files on D: along with the Acer restore stuff. I don't have my C110 yet (tomorrow!) but from what I've read this is just Acer "system restore" disk.
If you want to reclaim D:, you'll likely want/need to disable whatever restore software Acer is running. Otherwise what you lose is the ability to easily(?) recover in the event of some catastrophic failure. If you have the TCi (i.e. w/cd-dvd) or some other way to access a CD, you're probably safe for restoring the OS, but system restore may also save snapshots of runtime data you'ld rather not lose as well.