cat myfile.iso > /dev/hdb
Or, do you mean emulating that an iso is a CD, and not actually waste a hard drive as a cd? In which case, remove /dev/cdrom, copy the iso to /dev/cdrom, and then mount the iso. Applications usually look at /dev/cdrom as one of the common places for a cdrom (which is usually a symlink to /dev/hdc, or some other drive).
When finished, move /dev/cdrom to the original file, and make /dev/cdrom go to the cdrom drive again (ln -s /dev/hdc /dev/cdrom)
[ January 18, 2005: Message edited by: Stryker ]