Hi Mike
Having seen this problem several times I believe it is as simple as the gratuitous arp not reaching the external switches when a vm is migrated. The vSwitch i.e. the target host involved in the migration is supposed to send an rarp packet to the external switch. Due to HPs abysmal record of VC firmware updates I would bet the farm that the VC layer is dropping the rarp packet. Get VMware support to prove to you firstly that the rarp is being sent from the target host to notify the switches. Then get your network guys to see if they are recieving the rarp on the external switch ports. Either esxi is not sending the "Notify Switches" (rarp) packet at all or the VC layer is not passing it on to the blade uplinks (external switch ports). We have VC configured for tunneling (trunk mode) so this should not happen however we are still using VC assigned MAC addresses which have caused several major outages in the past. Every time networking breaks it is a combination of VC the emulex NICs or both.
The rarp packet theory is validated by the fact that if a VM is pinging a host on a different subnet or the gateway the problem never occurs during migration. Let me know what you find.
Cheers
Nick