Brett Yount from Microsoft's Bing Webmaster team has commented here and in a Bing Forum thread saying the issue with the fake referrers being generated by MSNBot has finally been fixed. By finally, I mean, an issue dating back over two years.
Thanks to our webmaster community and followers we are happy to share an update to the referrer issue that you may have heard or read about recently. First of all, we’d like to express our sincere apologies that this referrer issue continued past the August 20th date when we explicitly stated that it was fixed. With the support of many webmasters' data, our crawling team was able to pinpoint the root cause and deploy a new fix to stop the referrer string in production. This fix has been verified in all of our production beds.
There are confirmed reports by webmasters that the issue is no longer being seen and that going forward their log files are no longer being polluted with fake referrer data.
I am happy to hear reports that this is fixed, but you can't blame me for being a bit skeptical that this issue might creep up again. I mean, Microsoft fixed this issue at least three times in the past. I certainly hope it is fixed forever.
Forum discussion at Bing Forum.