I was upgrading a 10.0 system. I made a DVD from the 5CDs and I began to 
install it. Everything was fine. RAID was recognized, LVM also, no 
problem with upgrading from 10.0 to 10.1 Beta3, only a manual 
intervention for packages from community repositories (something 
normal). At the first reboot, unfortunately I got these:

md: md0 stopped
/init: line 479: 736 segmentation fault  /sbin/mdadm -- assemble --scan 
$mdconf
rootfs: major=9 minor=0 devn=2304
/dev/md0: unknown volume type
/init: line 512: =: command not found
Mounting root /dev/md0
mount: unknown filesystem type ''
umount: /dev: device is busy
umount: /dev: device is busy
Kernel panic - not syncing: Attemted to kill init!

After this, I tried to make a clean install. I formated the raid and 
install it again, again with no problems. At the first reboot, the same 
problem (the first reboot means before you set the root password, user 
accounts, networking, etc).