From the discussion on the Daz forums, it doesn't sound like there will be much backwards-compatibility at all due to the abandonment od Tri-Ax weight mapping and the dramatically different (and additional) bones and joints.

I really hate what Daz is doing, and it already looks like Genesis 2 support on the site is coming to a dead halt (the only genesis 2 products released today are hair that are primarily for Genesis 3)  This is my greatest fear.  I also expect to see less support for Gen3 by Rendo PAs in the future due to the complete lack of Poser compatibility.  For crying out loud, there's even a PA here who makes Poser-only dynamic clothing for Genesis 2!  He is one of the PAs who immediately come to mind when I say that DAZ has screwed Poser users over.

I also don't like what seems like the slow phasing out of 3Delight.  I don't feel like learning how to use yet ANOTHER render engine, plus my computer is 5 years old and probably can't handle it.

Dropping Poser support in favor of Max, Maya, and other 3D professional software truly disgusts me.  I speak as someone who doesn't even like Poser.  You don't take something away that you have always provided-- that's just rude and inconsiderate, and just downright vile thing to do.

I, too, do not feel like re-purchasing everything for yet another generation of figures.  Besides the fact that I suspect that Genesis 3 is also too high in the memory-usage category for my computer to handle.