Bug#248800: Acknowledgement (installer beta 4 installation report (mbr damaged, grub misdetection, cannot boot from rescue disk))
I think my guess about MBR damage was a false alarm. (but I've ruined
the evidence by this point).
When I repartitioned, the "active" boot partition flag was removed from
the XP partition. I restored it in the partitioning tool and that
allowed me to boot into XP again.
The partitioning tool allows you to get all of your partitions set up
before it warns you about errors. Required, but unspecified, properties
should probably be in a different color so it is more obvious.