Not to speak for cycling, the following is speculation:
QTkit is a steaming, complete fucking pile of horsehit. Ok, that may be a bit strong. It does some things very well, but does not let you get at low level internals easily, and while QTkit makes some things super straight forward, getting speed as well as full functionality out of it is somewhat maddening.
Im assuming Jitter is built using ye-olde Quicktime library, which does (almost) everything you could want, but is being replaced slowly by QTkit. QTkit wraps up some things in Quicktime (the library) and fixes some issues at the cost of not always exposing some of the deeper, lower level functionality, basically some things are Black Box you can't really get to, since bugs fixed in Quicktime (library) are done behind the scenes.
So, I imagine that some parts of Jitter would have to be re-written to use QTkit to gain the multi-DV compatibility, but I know for certain that QTkit does *not* expose natively some of the lower level stuff like IIDC camera settings dialog and things like that. On top of it, there are still some potential issues with multiple DV devices. Ive toyed a lot with QTKit and multi in and it is, in my experience, rather flakey.
Now, you can mix/match older Quicktime library + QTkit code to get around some of these issues, but I know there are gotchas and caveats when doing so. Add to the complications of the new 10.6 Quicktime X stuff (which changes QTkit behind the scenes even more), and it gets even nuttier.
But, remember, QTKit is (as far as I know), not available on Windows (No Cocoa apis although there is Obj-C), so that adds an additional complication since Jitter needs to support both platforms. In short, its complicated, and Quicktime in general is not heping.
Its really a shame about QTkit and Quicktime in general, it could be so awesome, yet fails when you get kind of deep into it if you need cross platform compatibility, and dont want to kill yourself.
Anyway, again, this is just speculation on the internals, I have no special insight except my experience.