How can I schedule a Nandroid backup? Is it possible to schedule a Nandroid backup for a particular time on a rooted Samsung Galaxy S2 running CM9 and the Siyah Kernel? How would one accomplish this?
You can make an automatic nandroid backup using ROM Manager (free / paid).

open the app - go to settings and select Automatic Backup.  You can select the time and frequency of the backups in the following screens.