The C++03 standard does not say anything about it. When you have no guarantees about the thread-safety of something, you should treat it as not thread-safe.
Of particular interest here is the fact that cout is buffered. Even if the calls to write (or whatever it is that accomplishes that effect in that particular implementation) are guaranteed to be mutually exclusive, the buffer might be shared by the different threads. This will quickly lead to corruption of the internal state of the stream.
And even if access to the buffer is guaranteed to be thread-safe, what do you think will happen in this code?
// in one thread
cout << "The operation took " << result << " seconds.";
// in another thread
cout << "Hello world! Hello " << name << "!";
You probably want each line here to act in mutual exclusion. But how can an implementation guarantee that?
In C++11, we do have some guarantees. The FDIS says the following in §27.4.1 [iostream.objects.overview]:
Concurrent access to a synchronized (§27.5.3.4) standard iostream object’s formatted and unformatted input (§27.7.2.1) and output (§27.7.3.1) functions or a standard C stream by multiple threads shall not result
in a data race (§1.10). [ Note: Users must still synchronize concurrent use of these objects and streams by
multiple threads if they wish to avoid interleaved characters. — end note ]
So, you won’t get corrupted streams, but you still need to synchronize them manually if you don’t want the output to be garbage.