What Happens to the String Data When Std: :String Objects Are Passed to Functions?

I've noticed something I don't understand happening to the string arguments to functions.

I've written this little test program:

#include <string>
#include <iostream>

using namespace std;

void foo(string str) {
  cout << str << endl;
}

int main(int argc, char** argv) {
  string hello = "hello";
  foo(hello);
}

I compile it like this:

$ g++ -o string_test -g -O0 string_test.cpp

Under g++ 4.2.1 on Mac OSX 10.6, str inside foo() looks the same as it does as hello outside foo():

12    foo(hello);
(gdb) p hello
$1 = {
  static npos = 18446744073709551615, 
  _M_dataplus = {
    <std::allocator<char>> = {
      <__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, 
    members of std::basic_string<char,std::char_traits<char>,std::allocator<char> >::_Alloc_hider: 
    _M_p = 0x100100098 "hello"
  }
}
(gdb) s
foo (str=@0x7fff5fbfd350) at string_test.cpp:7
7     cout << str << endl;
(gdb) p str
$2 = (string &) @0x7fff5fbfd350: {
  static npos = 18446744073709551615, 
  _M_dataplus = {
    <std::allocator<char>> = {
      <__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, 
    members of std::basic_string<char,std::char_traits<char>,std::allocator<char> >::_Alloc_hider: 
    _M_p = 0x100100098 "hello"
  }
}

Under g++ 4.3.3 on Ubuntu, however, it doesn't:

12            foo(hello);
(gdb) p hello
$1 = {static npos = 18446744073709551615, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x603028 "hello"}}
(gdb) s
foo (str={static npos = 18446744073709551615, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x7fff5999e530 "(0`"}}) at string_test.cpp:7
7           cout << str << endl;
(gdb) p str
$2 = {static npos = 18446744073709551615, _M_dataplus = {<std::allocator<char>> = {<__gnu_cxx::new_allocator<char>> = {<No data fields>}, <No data fields>}, _M_p = 0x7fff5999e530 "(0`"}}
(gdb) p str->_M_dataplus->_M_p
$3 = 0x7fff5999e530 "(0`"

So, what's happening to the value of the string when it is passed to this function? And why the difference between the two compilers?