This is just Google-fodder:
If you use the lenny (beta2) installer to setup a system which uses / on LVM on dm_crypt on raid, then the first reboot fails with something like “cannot find LVM volumes”. After a few minutes you get dumped into a busybox shell.
Getting the system to boot
cryptsetup luksOpen /dev/md1 md1_crypt (or md0, depending on your setup). Exit busybox and the system should boot through. Then …
Fixing this for good
It seems to be that this Wiki page still applies. Thus you have to create a line in /etc/crypttab containing (in my case md1)
md1_crypt /dev/md1 none luks
update-initramfs -u to update the initrd.