If temporaries are implicitly non-modifiable, how does this work?

I'm told that, in C++03, temporaries are implicitly non-modifiable.

However, the following compiles for me on GCC 4.3.4 (in C++03 mode):

cout << static_cast<stringstream&>(stringstream() << 3).str();

How is this compiling?

^{(I am not talking about the rules regarding temporaries binding to references.)}