mount: could not find any free loop device

You can see what is being used by a loop device with losetup:

  # losetup /dev/loop0
  /dev/loop0: [fd06]:234921356 (/linux/isos/backtrack.iso)

It is possible to increase the number of available loop devices. Free
all loop devices, and add a line with the following to

     options loop max_loop=64

(maximum is 256)

Then, do:  
     rmmod loop && modprobe loop

If you get an error that the module couldn't be removed, you still have loop 
devices in use.

Newer kernels (2.6.21 or 2.6.22) use a dynamic allocation of loop devices, 
so you will only have to create the filesystem representation of the devices:

  for ((i=8;i<64;i++)); do
    [ -e /dev/loop$i ] || mknod -m 0600 /dev/loop$i b 7 $i