Hmmm ... a maneira correta de fazer isso é:
dd if=/dev/zero of=./filesyst bs=10485760 count=1
sudo losetup /dev/loop0 ./filesyst
sudo mkfs.ext3 /dev/loop0
sudo mount /dev/loop0 /tmp/lalla
e funciona:
(0)romano-asus:~/tmp% ls -l /tmp/lalla
total 12
drwx------ 2 root root 12288 2011-12-20 22:21 lost+found
(0)romano-asus:~/tmp% df /tmp/lalla
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/loop0 9911 1121 8278 12% /tmp/lalla
Você deve liberar o primeiro dispositivo de loop com losetup -f
.