after further testing, it seems i can no longer boot from md0
after md0 was resync'd with sda & sdb plugged in together, i rebooted and started getting errors after loading the kernel such as missing files and segmentation faults.
so i unplugged sdb again, and guess what?? i booted fine! so does this prove that raid arrays cannot be reconstructed when one of the drives are already mounted? or is there something more sinister going on here?
edit: yes, i did wait until md0 completed resyncing before i hit reboot...