Today I was skimming some http log files for a particular page of mine. In the last hour, I received 50 visitors (unique IP addresses), but only 5 had referring data. Looking a little closer, the ones without the referring data were all using Internet Explorer on Windows and had "FunWebProducts" within their user agent.
So I did some Googling and FunWebProducts is another one of the million or so hijackware/adware/spyware toolbars that users unknowingly install on their computer. The interesting thing about these guys is they infected a very large percentage of the Internet's users very quickly. And now that I know what they are, I recall seeing their banner ads all over the place (including yahoo.com).
It doesn't seem to do a complete block of referral URL, just external ones (you will see image requests have a referral URL of your page for example). But then again, external referrals is the more important ones. I probably would have never cared or noticed how widespread the infection is if they didn't block the referral data. In truth, it's probably not blocking referrals directly, and instead does not show the referral URL of people using the search function it installs. I'm not going to install it to confirm though, sorry. :)
So now the assignment for all the readers... check your web logs and see what percentage of your visitors are not giving you referral data. You might be surprised how many of them have FunWebProducts within their user agent.