will we still have that ability [...] to interface with existing C libraries
Of course we will, and it will be much, much easier. For starters, check out Parrot's Foreign Library Access. You can open a shared library, DLL or whatever, and then just start using it. Parrot already has a fair amount of smarts to let you specify the calling conventions. Many XS problems will simply vanish because of the level of support that Parrot provides intrinsically.
And if Parrot can do that, you can be sure it will be easy to tap from Perl. And it will be Good.
- another intruder with the mooring in the heart of the Perl