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errno is usually implemented as thread local variable, more precisely as a macro that calls a function which returns the memory address of a thread local variable. Each C runtime library has its own function and TLS which introduces an issue when a Python extension uses a different CRT than the core. AFAIK that an issue only an issue on Windows with other versions of Visual Studio.
This means that mixed CRTs break the three PyErr_SetFromErrno* functions as they always access the errno of the core's CRT. The patch adds a Py_errno macro that must be used in extensions before a PyErr_SetFromErrno() function is used.
Example:
fd = open(filename, O_RDONLY);
if (fd == -1) {
 Py_errno = errno;
 return PyErr_SetFromErrnoWithFilename(PyExc_OSError, filename);
}
The issue was discovered by me a about two weeks ago and further analyzed by Martin. http://mail.python.org/pipermail/python-dev/2012-August/121460.html 

Since this is a new feature, it needs to go into 3.4.

I'd prefer to have new variants of the PyErr_SetFromErrno* functions where the errno value is explicitly passed in instead of adding a side-channel for passing in the value.

Agreed with Ronald.

Am I imagining things or have I read that the Windows CRT is going to remain stable in the future, meaning this work would no longer be needed.

Also agreed with not exposing a side-channel to set errno.
I'd expect this to no longer be an issue with the stable CRT, but I'm not 100% confident about saying that yet. In theory, there will only ever be one CRT loaded in the future, but there's probably going to still be situations where explicitly providing errno is necessary.

It's no longer a problem with new VS.