My host application is cubase sx3 so I'm not sure about that but I know cubase 5 is compatible and I've been meaning to upgrade to that for some time now. Also I'm a bit unclear on the d0 stepping I think that has more pottential to overclock right? but it doesn't seem to be specified on the adverts as I've not really seen it but only heard about it. All I seem to find is the general i7 920 chip, that's it.
Cubase SX3 does support multi-processing, although many with Q6600's at the time were not getting much more performance over the E6xx dual cores, so it is uncertain as to whether a Quad is fully efficient. After all, Cubase SX3 was released in 2005, which is believe was quite a bit before the first consumer quad processor. However, Cubase 5 does fully support up to 16 threads (we use it with a dual quad mac pro in the studio at work)
As for the D0 stepping, your correct in what you say. Most CPU's are released with a certain revision, and after a while, are released with a newer, more updated one which usually allows for lower temperatures and more overclocking ability. (Q6600's had the G0, E8xxx series had the E0). If you take a look in the overclocking forum, you'll find a few folk have their i7 920's past 4.0ghz, although most are on water i believe. However, with good case/cpu cooling, i bet you'd get 4.0ghz or thereabouts on air.