Bug#632401: Same issue here.

[Jose I. Gonzalez Goñi]

Same issue here, after moving my squeeze installation from a non-raid to 
a software raid1 (mdadm) configuration.
I found out that the file "md" located in initrd (conf/conf.d/md) does 
not get updated to reflect changes.
I created a new installation and created a software raid1 from the 
partition editor in the installation process.
After finishing the installation I compared the new initrd with the old 
one and found out that the md file in the new initrd had some info about 
the raid that the old one didn't.
Copied that filed into the old initrd, modified the setting to match the 
config on the server and the raid system booted fine.
So I guess the bug is related to update-initramfs not updating that file.
I hope this info helps and I you require any further details, please let 
me know.

Regards,
José
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