VMware introduced support for iSCSI back in the ESX 3.x days. However, ESX could only boot from an iSCSI LUN if a hardware iSCSI adapter was used. Hosts could not boot via VMware's iSCSI driver using a NIC with special iSCSI capabilities.

It quickly became clear that there was a need for booting via Software iSCSI. VMware's partners are developing blade chassis containing blade servers, storage and network interconnects in a single rack. The blades are typically disk-less, and in many cases have iSCSI storage. The requirement is to have the blade servers boot off of an iSCSI LUN using NICs with iSCSI capabilities, rather than using dedicated hardware iSCSI initiators.

In ESXi 4.1, VMware introduced support for booting the host from an iSCSI LUN via the Software iSCSI adapter. Note that support was introduced for ESXi only, and not classic ESX.

--> Please see the rest on the blog : http://blogs.vmware.com/vsphere/2011/11/how-to-configure-esxi-to-boot-via-software-iscsi.html