I was testing a site locally with opera 11.52 and I was surprised to see Opera won't send cookies out to IP style domains.
So I was setting up [127.0.0.1...] to do some testing for a site and then I was checking the $_COOKIE var on the server end. It was empty regardless, meaning opera never sent the cookies back to the server. If I change it to http://www.example.com which also is routed locally to 127.0.0.1 then there was no problem.
I think this is happening with the all numeric ips not just the local ones and it's irrelevant of web application.