If you're running Linux or Windows you can use software RAID1 (aka mirroring) over IP (or iRAID) from the OS side. On Linux it's relatively painless and much better if you use LVM's mirroring over Linux MD. On the Windows side you have to be supremely careful about using Dynamic Disks over iSCSI, but there's some magic that makes it works so well I don't know how I'd do without.

Like upgrading the iSCSI volumes from 1.1 to 1.3 without any user downtime. Sigh...

For VMware ESX HA/DRS I think it will work with either NFS or iSCSI - though I've yet to test it. NFS is almost as fast and much easier to manage than iSCSI for ESX volumes, plus you have the added benefit of having Open-E back them up automatically. In tests with more than 15 VM's accessing the same NFS volume, NFS actually pulls ahead of iSCSI due to the locking behaviour of NFS versus iSCSI.