I thinkI've found it
this is 1.0b1:
HTTP/None: 401 Unauthorized
here is 1.0a2
HTTP/None 401 Unauthorized
IE doesn't like the 'HTTP/None: 401'  it wants "HTTP/None 401 ..."
                              ^
this fixes the problem
148c148
<         append("HTTP/%s: %s" % (self._http_version, status))
---
>         append("HTTP/%s %s" % (self._http_version, status))
155a156
-steve