Although NULL IS ugly, it cannot be replaced with 0 (like others have explained). But you can always "rename" stuff in C/C++, so while it has been long since I last came across a compiler without support for nullptr, I would just redefine NULL as nullptr since I would probably also build the source on C++0x/C++11 complaint compilers, Of course, you loose the type of nullptr when compiling for pre-C++0x (std::nullptr_t) but with a little care (only load/store and compare with pointers), I cannot see why it would cause trouble.
#define nullptr NULL
EDIT: An enhanced version might look like this, since it does not pollute the global namespace and even gives nullptr a type:
const void *nullptr = (unsigned)NULL;