I noticed that it was possible to specify arbitrary arguments to 
io.BytesIO:

>>> io.BytesIO(b'foo', foo=1)
<_io.BytesIO object at 0x430150>

But io.StringIO doesn't:

>>> io.StringIO('foo', foo=1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'foo' is an invalid keyword argument for this function

So, I've attached a little patch that uses the same technique 3.1a1 uses 
to parse the arguments. One thing to be aware of is that I named the 
kwarg "intial_bytes" just like http://docs.python.org/3.0/library/io.html#io.BytesIO instead of 
"inital_value" like io.StringIO. I'm not sure if it'd be desirable to be 
consistent between the two.

Committed in r74316.

Thanks!