I know a few customers have done some pretty cool awareness videos that briefly highlight the "what's new" stuff. If you've got the budget, sticky notes/posters/table tents advertising what's new can be used in meeting rooms, bathrooms, inside the refrigerator in the break room, you name it.
Another idea is to feature a new capability each week, in the context of using it for real work.
For example, with the new rich microblogging, you could highlight the ability to mention others, and focus on how that might help spread awareness of whatever it is you microblogged. This is similar to the phenomenon of cc:'ing somebody in Twitter, so that they're sure to see whatever you Tweet.
For place admins, maybe highlight the awesome View Document widget (I can't remember if that was available in 4.0x) as a replacement for the Richt Text Widget - you can upload images in a document, manage the document editing, etc., and use the View Doc widget to display it.
For group admins in particular, I LOVE the Send Email to Group feature.
Hope this helps!