I assume this means they've found more site pages to put ads on, or stuffed more ads onto existing pages. I haven't seen where yet, so it's probably not all that noticeable during regular operations.
I'm not sure what ads in notification emails will mean (although I remember the ad text in notifications for sponsored gifts to Plus and Basic users), but will test and update this post when they seem to appear on arriving comment notification emails. Of course, this is to Plus users, so caveat emptor and all that--they have accepted ads in exchange for bling.
"Enabling graphical previews for all Basic/Plus users who haven't explicitly turned it off yet."
Ad revenue possibilities for enabling Snap.com shots were discussed here. To LJ's credit, however, people were given the chance to turn it off on their journal before it got turned on by default. Unfortunately, there is no way to opt out of it site wide unless you let snap.com set a cookie. But AdBlock Plus or NoScript can take care of that.