Well, I forgot to mention it in my previous message, but there is already a warning that you can activate with the -b option:

$ ./python -b
Python 3.3.0a0 (default:6b6c79eba944, Dec  6 2011, 11:11:32) 
[GCC 4.5.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> str(b"")
__main__:1: BytesWarning: str() on a bytes instance
"b''"


And you can even turn it into an error with -bb:

$ ./python -bb
Python 3.3.0a0 (default:6b6c79eba944, Dec  6 2011, 11:11:32) 
[GCC 4.5.2] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> str(b"")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
BytesWarning: str() on a bytes instance


However, -b is highly unlikely to become the default, for the reasons already explained. It was mainly meant to ease porting from Python 2.