I assume this is referring to the issue regarding visited links and browser history. This is a widespread, common, and ancient issue in web browsers. Only recently have some of them began fixing this issue.
First off, some background. A site defines different colors/images for links. One color/image for a regular default, another for when you mouseover the link, and another for when you've already visited that link in the past. This problem lies with the latter of those options.
Now, the server doesn't actually get a list of links you've visited, that would be crazy. Instead, the browser checks its cache, and modifies what it sees based on how the server has defined visited links. However, that doesn't mean the server can't exploit that information. If the server were to set a specific picture for a link, it wouldn't get downloaded by the client unless they had already been to that URL. Therefore, if the webserver logs a hit for a visited link image, it knows you've been to that URL. If the server were to give you a massive list of popular websites, each with a unique visited link, then it would effectively harvest your cache, and know where you've been.
This sounds like it'd only be marginally useful, but you would be amazed. As an example, check out This site. It can generate a pretty exhaustive list of places you've been. And this place uses that information to determine your gender. (It totally pegged me, with 97% certainty) I'm willing to bet that is what your wife had seen. I wouldn't be surprised that many mobile browsers are capable enough to fall victim too.
Here are some more sites that exploit this issue:
http://whattheinternetknowsaboutyou.com/all -- This page seems hokey at the moment, I was sure it worked before, though.
http://caughtyouwatching.com/ -- Determines if you've been to porn sites. (My work machine passed, phew!)
http://www.haveyourfriendsbeenthere.com/ -- This one has a link you can send to your friends, to see what they've been using. That's creepy as hell. I waffled on including this, as it seems so blackhat-ish. But it's probably worth noting, just as a defensive measure.
Here are some articles discussing the issue:
Firefox's bugzilla entry from....2002. I told you this was ancient!
Firefox's blog about fixing this issue, all of three months ago.