When the loop module hasn't already been loaded, this fails:

sudo grml-debootstrap --vmfile --target qemu.img -t wheezy --arch amd64 --filesystem ext3 --force 
--scripts ./util/build/post-scripts



 * grml-debootstrap - Please recheck configuration before execution:

   Target:          wheezy
   Install grub:    yes
   Using release:   squeeze
   Using mirror:    http://cdn.debian.net/debian
   Using hostname:  grml
   Deploying as Virtual Machine.
   Using Virtual Disk file with size of 2G.

   Important! Continuing will delete all data from wheezy!
 * Skip user acknowledgement as requested via --force option.
Formatting 'wheezy', fmt=raw size=2147483648
0 @0x168: 63 @4 (0x4)
 - 0x168: 32 (0x20) @ 4 (0x4)
 - 0x16A: 31 (0x1F) @ 36 (0x24)
using 2 blocks, 4 bytes (146 free)
using sectors of 2^9 = 512 bytes
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.0135541 s, 37.8 kB/s
64+0 records in
64+0 records out
64 bytes (64 B) copied, 7.9024e-05 s, 810 kB/s
 * Device-mapper not ready yet, trying to load dm-mod module.
mount: could not find any device /dev/loop#Bad address
can't set up loop
 * Error setting up loopback device.
 -> Failed (rc=1)
 * Removing /var/cache/grml-debootstrap/stages_wheezy
 * Removing loopback mount of file wheezy.