[Desire Android Development] sibere:
ext3 loopfile mounted without issues. should not be a problem to get it working on a partition!

sh-3.2# tune2fs -O has_journal testfile
tune2fs -O has_journal testfile
tune2fs 1.41.10 (10-Feb-2009)
Creating journal inode: done
This filesystem will be automatically checked every 33 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
sh-3.2# tune2fs -l testfile
tune2fs -l testfile
tune2fs 1.41.10 (10-Feb-2009)
Filesystem volume name: <none>
Last mounted on: <not available>
Filesystem UUID: 0f556305-5534-4519-b964-e952683a190b
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index filetype sparse_super
Filesystem flags: unsigned_directory_hash
Default mount options: (none)
Filesystem state: clean
Errors behavior: Continue
Filesystem OS type: Linux
Inode count: 5136
Block count: 20480
Reserved block count: 0
Free blocks: 18626
Free inodes: 5125
First block: 1
Block size: 1024
Fragment size: 1024
Reserved GDT blocks: 79
Blocks per group: 8192
Fragments per group: 8192
Inodes per group: 1712
Inode blocks per group: 214
Filesystem created: Fri Apr 22 13:33:14 2011
Last mount time: n/a
Last write time: Fri Apr 22 13:37:15 2011
Mount count: 1
Maximum mount count: 33
Last checked: Fri Apr 22 13:33:14 2011
Check interval: 15552000 (6 months)
Next check after: Wed Oct 19 13:33:14 2011
Reserved blocks uid: 0 (user root)
Reserved blocks gid: 0 (group root)
First inode: 11
Inode size: 128
Journal inode: 8
Default directory hash: half_md4
Directory Hash Seed: a64450c2-bda3-4731-a535-b65ca8537a5a
Journal backup: inode blocks
sh-3.2# busybox mount -o loop -t ext3 testfile /data/perso/sdext
busybox mount -o loop -t ext3 testfile /data/perso/sdext
sh-3.2# busybox df -h
busybox df -h
Filesystem Size Used Available Use% Mounted on
tmpfs 202.3M 64.0K 202.2M 0% /dev
tmpfs 202.3M 0 202.3M 0% /mnt/asec
tmpfs 202.3M 0 202.3M 0% /mnt/obb
tmpfs 8.0M 0 8.0M 0% /app-cache
/dev/block/mtdblock3 250.0M 210.7M 39.3M 84% /system
/dev/block/mtdblock5 147.6M 102.7M 44.9M 70% /data
/dev/block/mtdblock4 40.0M 1.1M 38.9M 3% /cache
/dev/block/vold/179:1
6.7G 4.1G 2.6G 62% /mnt/sdcard
/dev/block/vold/179:1
6.7G 4.1G 2.6G 62% /mnt/secure/asec
/dev/loop0 19.4M 1.2M 18.2M 6% /data/perso/sdext
sh-3.2# busybox mount
busybox mount
rootfs on / type rootfs (ro,relatime)
tmpfs on /dev type tmpfs (rw,relatime,mode=755)
devpts on /dev/pts type devpts (rw,relatime,mode=600)
proc on /proc type proc (rw,relatime)
sysfs on /sys type sysfs (rw,relatime)
/sys/kernel/debug on /sys/kernel/debug type debugfs (rw,relatime)
none on /acct type cgroup (rw,relatime,cpuacct)
tmpfs on /mnt/asec type tmpfs (rw,relatime,mode=755,gid=1000)
tmpfs on /mnt/obb type tmpfs (rw,relatime,mode=755,gid=1000)
tmpfs on /app-cache type tmpfs (rw,relatime,size=8192k,mode=755,gid=1000)
none on /dev/cpuctl type cgroup (rw,relatime,cpu)
/dev/block/mtdblock3 on /system type yaffs2 (ro,relatime)
/dev/block/mtdblock5 on /data type yaffs2 (rw,nosuid,nodev,relatime)
/dev/block/mtdblock4 on /cache type yaffs2 (rw,nosuid,nodev,relatime)
htcfs on /data/htcfs type fuse.htcfs (rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other)
/dev/block/vold/179:1 on /mnt/sdcard type vfat (rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,al
low_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)
/dev/block/vold/179:1 on /mnt/secure/asec type vfat (rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=07
02,allow_utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro)
tmpfs on /mnt/sdcard/.android_secure type tmpfs (ro,relatime,size=0k,mode=000)
/dev/loop0 on /data/perso/sdext type ext3 (rw,relatime,errors=continue,barrier=0,data=writeback)

and for even more safety for the moment

sh-3.2# busybox umount -d /data/perso/sdext
busybox umount -d /data/perso/sdext
sh-3.2# busybox mount -o loop -t ext3 -o data=ordered,barrier=1 testfile /data/perso/sdext
<op -t ext3 -o data=ordered,barrier=1 testfile /data/perso/sdext
sh-3.2# mount
mount
rootfs / rootfs ro,relatime 0 0
tmpfs /dev tmpfs rw,relatime,mode=755 0 0
devpts /dev/pts devpts rw,relatime,mode=600 0 0
proc /proc proc rw,relatime 0 0
sysfs /sys sysfs rw,relatime 0 0
/sys/kernel/debug /sys/kernel/debug debugfs rw,relatime 0 0
none /acct cgroup rw,relatime,cpuacct 0 0
tmpfs /mnt/asec tmpfs rw,relatime,mode=755,gid=1000 0 0
tmpfs /mnt/obb tmpfs rw,relatime,mode=755,gid=1000 0 0
tmpfs /app-cache tmpfs rw,relatime,size=8192k,mode=755,gid=1000 0 0
none /dev/cpuctl cgroup rw,relatime,cpu 0 0
/dev/block/mtdblock3 /system yaffs2 ro,relatime 0 0
/dev/block/mtdblock5 /data yaffs2 rw,nosuid,nodev,relatime 0 0
/dev/block/mtdblock4 /cache yaffs2 rw,nosuid,nodev,relatime 0 0
htcfs /data/htcfs fuse.htcfs rw,nosuid,nodev,relatime,user_id=0,group_id=0,allow_other 0 0
/dev/block/vold/179:1 /mnt/sdcard vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_utime
=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
/dev/block/vold/179:1 /mnt/secure/asec vfat rw,dirsync,nosuid,nodev,noexec,relatime,uid=1000,gid=1015,fmask=0702,dmask=0702,allow_
utime=0020,codepage=cp437,iocharset=iso8859-1,shortname=mixed,utf8,errors=remount-ro 0 0
tmpfs /mnt/sdcard/.android_secure tmpfs ro,relatime,size=0k,mode=000 0 0
/dev/loop0 /data/perso/sdext ext3 rw,relatime,errors=continue,barrier=1,data=ordered 0 0