stringstream, string, and char* conversion confusion

My question can be boiled down to, where does the string returned from stringstream.str().c_str() live in memory, and why can't it be assigned to a const char*?

This code example will explain it better than I can

#include <string>
#include <sstream>
#include <iostream>

using namespace std;

int main()
{
    stringstream ss("this is a string\n");

    string str(ss.str());

    const char* cstr1 = str.c_str();

    const char* cstr2 = ss.str().c_str();

    cout << cstr1   // Prints correctly
        << cstr2;   // ERROR, prints out garbage

    system("PAUSE");

    return 0;
}

The assumption that stringstream.str().c_str() could be assigned to a const char* led to a bug that took me a while to track down.

For bonus points, can anyone explain why replacing the cout statement with

cout << cstr            // Prints correctly
    << ss.str().c_str() // Prints correctly
    << cstr2;           // Prints correctly (???)

prints the strings correctly?

I'm compiling in Visual Studio 2008.