Do you plan to port all the new stuff to ppc so that PB can produce universal binaries, or do you want to do x86 only which should be much easier (Putting Mach-O support into the normal x86 version and recompiling and adjusting the OS X libs to pb 4). I think for the libs themselves it shouldn't matter should it?
I know, I will be using c++ on my Intel Mac, when I get it in july, for sure. However having PureBasic to do the little tasks much faster and to use existing code, gives it even a better taste