could not reproduce:

Python 2.6.5 (r265:79096, Mar 19 2010, 21:48:26) [MSC v.1500 32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> from cStringIO import StringIO as F
>>> import json
>>> json.dumps('foo')
'"foo"'
>>> f=F()
>>> json.dump('foo',f)
>>> f.seek(0,0);f.read();f.seek(0,0)
'"foo"'
>>> json.dump({'a':['b']},f)
>>> f.seek(0,0);f.read();f.seek(0,0)
'{"a": ["b"]}'
>>> 


Python 2.6.5+ (release26-maint, Jul 30 2010, 23:04:10)
[GCC 4.4.4] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import json
>>> from cStringIO import StringIO as F
>>> f=F()
>>> json.dump('foo',f)
>>> json.dumps('foo')
'"foo"'
>>> f.seek(0,0);f.read();f.seek(0,0)
'"foo"'
>>> json.dump({'a':1,'_':2},f)
>>> f.seek(0,0);f.read();f.seek(0,0)
'{"a": 1, "_": 2}'
>>> json.dumps({'a':1,'_':2})
'{"a": 1, "_": 2}'
>>>