ok, I nave completed setting up a test environment with DDP and it seems to work well with multipathing.
The setup is
MD3660i
Controller 0
192.168.130.101
192.168.131.101
Controller 1
192.168.130.102
192.168.131.102
So two subnets across two controllers.
The controllers are attached to two dedicated switches, one for each subnet.
ESXi5.1
Create vSwitch1 with one vmkernel 192.168.130.103 with a 1:1 ratio to one vnic
Create vSwitch2 with one vmkernel 192.168.131.103 with a 1:1 ratio to the other vnic
Plug the nics into the relevant switch for each subnet.
Add the iSCSI SW adapter. DO NOT port bind the vmks as you should not do this with multiple subnets.
Add to the dynamic discover tab of the iSCSI SW adapter -
192.168.130.101
192.168.131.101
192.168.130.102
192.168.131.102
MD3660i
Create a disk pool (DDP) with a Virtual Disk.
Create a host group with a host with the iqn of the ESX iSCSI SW adapter.
Map the Virtual disk to the host (LUN mapping)
ESXi5.1
Enable ALUA ( Asymmetric Logical Unit Access) on the CLI of the ESX host (ssh)
VMware Compatibility Guide: Storage/SAN Search
esxcli storage nmp satp rule add -s VMW_SATP_ALUA -V DELL -M MD36xxi -c tpgs_on;
esxcli storage nmp satp set --default-psp VMW_PSP_RR --satp VMW_SATP_ALUA;
Rescan the iSCSI SW adapter and there is your LUN!
I tried various multipath/ failover scenarios by unplugging cables, changing preferred controllers etc and all worked perfectly.