No, iSCSI off course not \:\)
But in my Inventory I always use $obj.Model from Win32_ComputerSystem.
That returns "VMware Virtual Platform" on ESXi 3.5, 4.0, 4.1, 5.0 and 5.0u1 as well as on Workstation and Fusion. It is also the only thing I could find that would stil work after a P2V conversion.

But keep in mind, this of course gives a different value when using Xen, Hyper-V or RedHat's Hypervisor crap. Different Hypervisor different value.