So I ran into this one today on RHEL 6.0 when trying to mount up a few additional isos on one of my remote kickstart servers. I have actually run into this issue before in RHEL 5 ( see here) however being that this particular kickstart server was built using RHEL 6.0 and not RHEL 5 I figured that there was probably a new process to resolve as RHEL 6 no longer uses the traditional /etc/modprobe.conf.
Note that a loop device is a pseudo device that makes a file accessible as a block device. In this case, the iso is the file.
So here is how you fix it.
First run the following command to increase the total number of loop devices to the max number of 256.
MAKEDEV -v /dev/loop
Then you need to make sure that this command runs at boot time, so stick it in /etc/rc.local. I think you can also pass this kernel option to the OS at boot in the /etc/grub.conf however I have not tried it.