The short answer is, "maybe". Python will be CPU bound but not I/O bound. This means you can have multiple threads concurrently performing I/O. On the other hand, if you have two threads which are CPU bound, only one will run at a time.
Having said that, there are plenty of ready work arounds. One is to make an extension module which executes your CPU bound tasks while releasing the GIL. The other is to break your script into a multiprocess model rather than a multithreaded module; using IPC as needed. -- http://mail.python.org/mailman/listinfo/python-list