I use software raid (mdadm) and do NOT use lvm (but lvm is installed automatically with Ubuntu Dapper Server Edition).
After booting, md0, md1, and md2 are created and mounted perfectly. However, two extra devices are created (and not mounted because they are not in fstab): md3 and md4.
So, md8 and md6 are equal; so is md5 and md7. Please note that the devices of md5 and md6 are hda/hdc as it should; but md7 and md8 are using 'dm-1, dm-0, dm-3 and dm-2' which is strange (to me).
I disabled the lvm boot script in /etc/init.d, but to no avail.
after next reboot, all was fine, and no weird 'dm' devices where created anymore.
Thanks too Frogzoo at #ubuntu for thinking together with me.