It's beginning to look like it might just have been a mistake on Amazon's part but it's a worrying occurrence, mistake or not. (Publisher's Weekly has info on the glitch and the problem and, thanks to a Neil Gaiman update I think this covers the reasons - or potential reasons - nicely.)
From what I can make out it seems concentrated on Amazon.com, where various titles with suppoosed adult content, particularly those with a LGBT content (including ridiculous entries - I saw Ellen DeGeneres' autobiography was included in the ban), are still available on the Amazon site but are being excluded from best-seller lists, have their Amazon sales ranks removed and generally find themselves less visible on the site than other, more wholesome works (such as Playboy centerfold books and the like).
I first noticed it, as I'm sure, many others did, via Neil Gaiman's twitter :
"Oranges are Not the Only Fruit AND The Well of Loneliness both lose their Amazon ranking? Complete and utter #amazonfail http://bit.ly/JoVF"Since then, #amazonfail has made top trend in twitter and #amazonfail on google gives you a lot of results right now. I think Amazon will be having a lot of PR meetings today and I smell a very big apology coming tomorrow. Mistake or not, it's a bad thing. Amazon, as the premier online retailer should be much better than this, should be much more aware of the repercussions of their actions than this. The fall-out from this one will be around for a long time.
More info: here, here, here, here, here, here and a great article on why being a stupid company on the Internet is a really bad thing here.
Neil Gaiman's blog post on the subject: here.
There's a petition to sign here (11,000 and counting).
And the googlebombing has already started - Amazon Rank now returns this ahead of Amazon's own definition.