Thanks erico.
I studied that PDF myself, before I posted this question - sorry I didn't let you know, but I just don't get it.
Why should it be compatible with the Z640 but not the Z620? I just hoped and assumed that the reason the Zx20 series where not mentioned in the compability list was because these series are EOL now, but I most certainly didn't expect they not to support the G2 drives. These machines are still under warranty, not that I consider that to be a spesific reason for why they should be supported, but it states that the machines are not that old and are expected to last longer than they have today.
As far as I can see the Z Turbo Drive G2 is just an upgrade of the Z Turbo Drive, which is compatible with both workstations. Further, it seems that their both based on the same PCIe interface, so I just don't see any obvious reason why their not supported. The PDF state that the workstations" does require a BIOS update for any system shipped prior to the Z Turbo Drive G2 launch, minimum BIOS is 1.53" so I guess that the Z620 could need an BIOS update as well but is that all preventing these drives from being compatible?