Umm, no, actually I was thinking of K&R C. In "The C Programming Language (Second edition)" (I haven't the first handy) it says:

The symbolic constant NULL is often used in place of zero, as a mnemonic to indicate more clearly that this is a special value for a pointer.

It was variations on (void*)0 that I was alluding to as being a cause of grief in some rare cases (none of which I can think of at the moment, and probably all in a C++ context in any case).

Actually NULL is not defined by C++ either, athough in "The C++ Programming Language (Third Edition)" Stroustrup comments:

Because of C++'s tighter type checking, the use of plain 0, rather than any suggested NULL macro, leads to fewer problems.

then goes on to suggest:

If you feel you must define NULL, use: const int NULL = 0;

None of which is relevant to very much, except to help highlight how little of the what people think of as C is in fact part of the C language.