I've got a similar situation, with a little bit of a twist.
I've just migrated my FC8 system with 3-160GB drives to 2-750GB drives. Using the pvmove command, I was able to move all my LVM extents from the old drives to one of the new drives with no issues.
Now, I want to use the 2nd 750GB drive as a mirror, but cannot rebuild the system from scratch, as this would take about a week, with all the custom software I have running.
I can't seem to do this automatically since the segments from the 3 individual drives show up in the Redhat LVM manager as separate, and the LVM Manager wants 3 disks to configure a mirror.
So, I'm not sure if this is even possible, but I'd imagine I need to somehow rebuild or merge the 3 segments into one before I can configure the mirror.
I'm going to play with this for a while in a virtual let you know what I find out.
Falco, if you have any suggestions to start from, I'd apprediate it.