immutability and constness

[Minas Mina]

I'm fairly new to D, but I have been using C++ for a while now 
(about 5 years, selftaught).

 From what I have learned, const in C++ is inconsistent.

For example:

int main()
{
    const int x = 0; // could be placed in ROM because it's known 
at compile time
}

void f(int x)
{
   const int y = x * x; // logical const
   // blah blah blah
}

const can be also cast away (const_cast if I remember correctly - 
I wasn't doing it often :) ) and mess everything.

 From what I understand, immutable in D is truly immutable. But I 
have seen some code here on the forum that casts aways 
immutability... (Is that even defined behaviour? What if that 
thing was in ROM?)

D has const as well... This is were it becomes a bit tricky for 
me. To be honest, I haven't got the book about D - it should(does 
it?) have information about that.

Can someone explain what are D's goal about immutable, const - 
and why it is bitwise const and not logical? How does it benefit 
programs? I have read something about that in the "general 
discussions" forum but posted the question here because it's for 
me (and others) to learn about that.

Thanks.