Update: I’m an idiot. This is a feature that’s part of Lifehacker’s Better GReader Firefox extension that I installed the other day. I just now noticed the behavior, and though it was part of Google Reader itself. Still, a cool feature - check out Better GReader for that and more! And sorry for the false alarm. Here I thought I was breaking news on a new feature! 
Just noticed this - a new Google Reader feature!
When I clicked the post title, instead of opening in a new tab, as usual, it opened an inline preview of the entire target web page, right there inside Google Reader.
There’s a new “Preview” button at the bottom of the entry, too, where you can toggle the preview on and off.
Haven’t read anything from Google about this feature yet, but it’s neato! 
TinyScreenfuls Feed (this blog)
Must be new. I don’t have the feature or the “preview” link yet…
This is not a Google feature. You must have Better GReader Firefox extension installed and enabled. http://lifehacker.com/software/exclusive-lifehacker-download/trick-out-google-reader-with-better-greader-262020.php
Geez you guys are fast! Yes, I realized my mistake a couple minutes after posting, and updated. Oops! I feel stupid!
Still a neat feature, though, and a good reason to get and use Better GReader.
I love the “Better” extensions, but actually have that particular feature disabled. If I see something I like in a feed I click it to open a new tab in the background, then I go through and read all the pages I picked out of interest. Just different way of doing it.
I DO love the color coding for the article sources in the List View. It helps my scanning of articles enormously.