Embedding links in stories is a years-old practice for many news media, but the problem is that it usually takes a user off the site to never return. It's as if someone reading the sidebar to a main story in the newspaper put the paper down and left the rest of it unread.
The Washington Post is starting to tinker with that. It'll put links on its stories that can display whatever is at the destination when the cursor rolls over the link, and inside or adjacent to that preview box will be a paid ad.
The Post is introducing this through two of its blogs, The Fix and Celebritology, and is using Apture as the technical supplier. But it's an experiment many will watch.