GotW #88: Is it safe to const_cast a reference to a tem****ary?

Herb Sutter wrote a new Guru-of-the-Week column last month, GotW #88,
about binding a tem****ary object to a reference-to-const.  Now if this
tem****ary isn't const, is it safe to const_cast the reference?

#include <string>
#include <cassert>
using std::string;

  string f() { return "abc"; } 

  void h() {
    const string& s1 = f();
    const string& s2 = s1;
    const_cast<string&>(s1) = "new value!";  // Safe?
    assert(s1 == "new value!");  // Okay?
    assert(s2 == "new value!");  // Okay?
  }

See also:
GotW #88 - A Candidate For the "Most Im****tant const."
http://herbsutter.spaces.live.com/blog/cns!2D4327CC297151BB!378.entry

Kind regards,
-- 
Niels Dekker
http://www.xs4all.nl/~nd/dekkerware
Scientific programmer at LKEB, Leiden University Medical Center