I think we have this sorted.  Turned out to be a physical cabling issue.  Has now been running for 24 hours without any "path flapping".  Have been able to "redistribute disks" and get them to stay on their preferred path.
A week ago we updated our storage array (MD3200 dual controller) from v07.70 thru 07.75 (interim) to 07.80 firmware.  We also updated our three (3) x ESXi hosts to v4.1 U2.  After the upgrades we've had some hardware issues and are trying to sort them out.  But we have noticed problems with the MD3200 and I am uncertain whether it is hardware-related or a firmware issue.  Symptoms are as follows: I have tried the Redistribute Virtual Disks option in Storage Manager but it doesn't have any noticeable impact (Storage Manager shows controller 0 with all the virtual disks, controller 1 with no virtual disks).  I've tried removing and reinserting controller 1 (which is what the hba3 adapters are all connected to). Is anyone else running an MD3200 (shared SAS) dual controller with v07.80 firmware and ESXi 4.1 ... if you are, are you experiencing any pathing issues, or is everything behaving 100% normal? To me this is looking more and more like (a) faulty SAS HBA in ESX1; and/or (b) a faulty controller 1 in the MD3200 (perhaps the firmware update of that controller went wrong somehow)? EDIT:  before anyone asks ... MD3200 firmware v07.80 is now listed as compatible with ESXi 4.1 U2 on the VMware HCL
MD3200 (shared SAS) running 07.80 firmware with ESXi 4.1 U2