A few days ago, I was surprised to see that, according to Google Webmaster Tools, a lot of the pages on several of my sites were marked as "Not Selected" for indexing. Usually, that would indicate duplicate content, or that there are multiple URLs that can be used to access the same pages on your site.
I did a little digging to see whether WordPress was allowing access via different URL structures, and didn't find anything. Any anyway, I have WordPress displaying a canonical link tag on each page. So why would Google think I have a bunch of duplicate pages?
I may have found the answer.
In my httpd.conf, I was spelling some of my domain names in "camel case" (eg. WhiteHatCrew.com, instead of whitehatcrew.com). That shouldn't make any difference -- domain names aren't case sensitive. I would have thought that Google would treat both identically.
But the sudden drop in "Not Selected" pages on a few of my sites since switching to all lowercase suggests otherwise.
If you have uppercase letters in your domain names in httpd.conf (or a file loaded by httpd.conf), WordPress will use the same capitalization in your canonical links and other links between pages (that's how PHP gives the domain names to it).
It may just be a coincidence. Maybe Google figured something else out about my sites at almost exactly the same time that I made the change. But until I hear otherwise, I'm going to assume that they're not normalizing the case of domain names.
My advice -- use all lowercase domain names in httpd.conf.