This workds just fine:

{{{
curl -k --user myUsername:myPassword https://api.bitbucket.org/1.0/user/repositories/?format=xml\\
}}}

Same user/pass and uri from .NET fails:

{{{
WebRequest request = WebRequest.Create("https://api.bitbucket.org/1.0/user/repositories/?format=xml");
request.Credentials = new NetworkCredential("myUsername", "myPassword");
WebResponse response = request.GetResponse();
}}}

WebException: The remote server returned an error: (401) Unauthorized.