Backwards compatible with what? Currently builtin modules can't be in packages at all, so nothing's lost.
But DLLs *can* be (that's the whole point, no?). If the rules for the init func changes, I think at least Marc-Andre L. won't be too happy: all (?) of his extensions use DLLs as submodules, so he would need to add switches to remain compatible with 1.5.2. I'm sure he's not the only one.
I admit I hadn't thought about the possibility that someone might have used dynamic libraries in packages already; my development cycles always include statically linked modules at some stage, so all extension modules remain top-level.
Which makes me wonder how others develop extension modules: I always use a debugger at some point, and I haven't yet found one which lets me set breakpoints in dynamic libraries that haven't been loaded yet!