Most of the cases we ran into were metaprogramming techniques which test whether an expression is a valid null pointer constants. These got innocently applied to 'false' and trigger the warning needlessly.

Done that before...had a variable that was formerly an int:

  int foo = 0;

But got changed into a non-integral type:

  Thing foo = 0;

Turns out Thing had a copy constructor roughly like:

  Thing::Thing(Thing* old)
      : field(old->field)
  {
  }

Needless to say, my program was not very happy.

I think the -Wconversion-null warnings are the other way around:

    int i = NULL;

Because of C++'s conflation of NULL and 0 (and now C++0x's nullptr).