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This one is a bit strange:
>>> f = io.StringIO("a\r\nb\r\n", newline=None)
>>> f.read()
'a\nb\n'
>>> f = io.StringIO("a\r\nb\r\n", newline=None)
>>> f.read(6)
'a\nb\n'
>>> f = io.StringIO("a\r\nb\r\n", newline=None)
>>> f.read(5)
'a\n\nb\n'

This is fixed by the io-c branch merge. (r70152)

Although this was fixed for 3.1, it appears not to be fixed for Python
2.6 or 2.7.
PS C:\Users\jaraco> python
Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit
(Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> io.StringIO('foo\r\nbar\r\n', newline=None).read()
u'foo\n\nbar\n\n'
Note that this behavior is slightly different from what pitrou reported.
If this cannot be fixed in 2.6, the documentation should at least
reflect that it doesn't work until a later version.

The bug shouldn't affect 2.6 and 2.7 unless you backported the now
obsolete _stringio module from 3.0.
I tested 2.6 and 2.7 and as expected I didn't see the bug:
Python 2.6.2+ (release26-maint:72576, May 11 2009, 23:16:48) 
[GCC 4.3.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import io
>>> io.StringIO('foo\r\nbar\r\n', newline=None).read()
u'foo\nbar\n'

Perhaps I was wrong about 2.7. However, I'm using stock builds of Python 2.6.2 for Windows, both 32- and 64-bit, and I get the undesirable behavior. Apparently the problem is platform-specific. Should this issue go under a new ticket?

> Should this issue go under a new ticket?
Yes, it would be preferable as the issue is probably not specific to
io.StringIO. Also, make sure that you include the result of this test-case:
open("testnl.txt", "wb").write("foo\r\nbar\r\n")
open("testnl.txt", "rU").read()

I've filed the Windows issue as http://bugs.python.org/issue6127