Thanks for your comments. Here's what I did to recover.
1. shutdown all processes and databases using the array. lsof /dev/md0 is your friend.
2. Full backup, in addition to the usual nightly ones.
3. Stop the array mdadm -S /dev/md0
4. Added the drive back into the array. In this case,
mdadm /dev/md0 --add /dev/sdb1
5. Sit back and watch progress, watch -n 1 cat /proc/mdstat
6. Restart, dmesg says
raid1: device sdc1 operational as mirror 1
raid1: device sdb1 operational as mirror 0
raid1: raid set md0 active with 2 out of 2 mirrors
md: ... autorun DONE.
Leave work early and make the boss do Helpdesk, life is good