I wrote about this issue here: http://blogs.msdn.com/b/ieinternals/archive/2010/11/22/internet-explorer-post-bodies-are-zero-bytes-in-length-when-authentication-challenges-are-expected.aspx
Your question omits important information which makes it hard to troubleshoot. You should never see the problem described with the literal URLs you've used, because IE uses protection spaces to decide whether a site is going to demand credentials via a HTTP/401 and example.com/
and example.com/foo/
are different protection spaces.
It would be very helpful if you could share a Fiddler log of this scenario for better troubleshooting.