I wanted to remove /dev/hdk6 from my array and replace it with a new 
drive at /dev/hdj6 (I know about the efficiency problems involved with 
this, but trust me that this is what I want to do for some other 
reasons), so I did the following:

mdadm -f /dev/md6 /dev/hdk6
mdadm -r /dev/md6 /dev/hdk6
mdadm -a /dev/md6 /dev/hdj6

The RAID reonsctucted itself, and it was working fine, so I rebooted the 
machine and removed the /dev/hdk hard drive from my computer.  When 
linux started up again, I checked /proc/mdstat, and it said that the 
RAID was running in degraded mode without /dev/hdj6.

I did the process again and stored information at each step (you can see 
this in the tarball).

I am using mdadm 1.2.0.

Thanks for the help.

--jeremy