VMware have come back to me (in fact the author of one of the KBs I referenced)
He has confirmed that the terminology in the KB is misleading and by 'broadcast domain' he actually means 'subnet' So layer 3 not layer 2.
This means that you CANNOT have multiple subnets in one broadcast domain (VLAN) AND use iSCSI SW port binding.
BUT
He told me (he is very familiar with the Dell MD3660i iSCSI kit), you do not have to have port binding to achieve multi pathing and load balancing. If you have a requirement for multiple subnets from your iSCSI SAN vendor then just create multiple vmks on different subnets and DO NOT do port binding. The fact they are on different subnets will be enough to achieve multipathing
He is updating the KB to make all of this much clearer.
I hope this helps someone