Well, one would not want to run a realtime application under Classic, anyway. The reason is simple: it is my understanding that Classic is running as a non-realtime process under X, and will be pre-empted by the X scheduler.
The good news is that Mach 3 did have support for realtime (read: non-preemptive) processes, but I don't know whether Darwin is able to use or implements such support at this time. Co-operative multithreading on the system level is much more suitable for realtime applications, exactly because one thread can easily hog the system.
Well, what I am saying might as well be FUD as far as Darwin/MacOS X is concerned, because the foundation OS (Mach 3) was engineered with realtime support; the point is that I do not know whether this is currently used by the current implementation of Darwin/Mac OS X.
However, traditional unices are performing rather poorly in realtime aspects (Darwin is not a traditional unix in this sense), so I would not be shocked if there were some temporary problems caused by the BSD layer.
This should not be quite noticeable in Quicktime, apart from occasional frame drop: sound sync, if buffered, is easy; it's only 150 KBps, or thereabouts. 24 channel realtime harddisk recording with realtime effects is not as easy because sound discontinuities are much more noticeable than dropped video frames.
So, Cubase would have problems in a traditional unix, or at least most of them (FreeBSD stable would be quite useless, and Linux should not fare any better---Solaris might, though); whether it will have problems in Mac OS X remains to be seen (but it really should not, especially once DVD playback is there to prove that realtime aspects of Mac OS X are acceptable).
Not directly connected to this thread, but you can find folders named VST in one of the folders in de users directory. Don't know why it's there, but could be a hint. There's also a folder in the same location named DigiDesign.